Creamy Chicken Mushroom Casserole

Prep Time	20 minutes
Active Stovetop Time	20 minutes
Bake Time	25 minutes
Total Time	1 hour 5 minutes
Yield / Servings	6 generous servings
Difficulty Level	Easy to Intermediate
Stovetop Heat Intensities	Medium-High (searing), Medium (aromatics & sauce reduction)
Oven Temperature	375°F (190°C) conventional / 350°F (175°C) convection

Ingredients

For the Chicken & Mushrooms

• 2 lbs (900 g) boneless, skinless chicken breasts or thighs, cut into uniform 1-inch bite-sized pieces

• 1 ½ lbs (680 g) cremini (baby bella) mushrooms, wiped clean and sliced ¼-inch thick

• 2 tablespoons olive oil, divided

• 2 tablespoons unsalted butter

• 1 ½ teaspoons kosher salt, divided

• ¾ teaspoon freshly cracked black pepper, divided

For the Creamy Garlic-Herb Sauce

• 1 medium yellow onion or 2 large shallots, finely diced

• 4 cloves garlic, minced

• 1 tablespoon fresh thyme leaves (or 1 teaspoon dried thyme)

• 1 teaspoon fresh rosemary, finely chopped

• 3 tablespoons all-purpose flour

• ½ cup (120 ml) dry white wine (such as Sauvignon Blanc or Pinot Grigio) or additional chicken broth

• 1 ½ cups (360 ml) low-sodium chicken broth, warmed

• 1 cup (240 ml) heavy whipping cream, room temperature

• 1 tablespoon Dijon mustard (adds subtle brightness without tasting overtly like mustard)

• ¾ cup (75 g) freshly grated Parmigiano-Reggiano, divided

For the Golden Topping & Garnish

• 1 cup (100 g) shredded low-moisture mozzarella or Gruyère cheese

• ½ cup (30 g) panko breadcrumbs

• 1 tablespoon melted unsalted butter

• 2 tablespoons fresh flat-leaf parsley, finely chopped

Step-by-Step Instructions

1. Prep and Preheat (5 minutes | Oven at 375°F / 190°C)

Position a rack in the center of your oven and preheat it to 375°F (190°C). Lightly grease a 9×13-inch (23×33 cm) ceramic or glass baking dish with butter or nonstick spray. Pat the diced chicken dry with paper towels and season evenly with ¾ teaspoon of the kosher salt and ½ teaspoon of the black pepper. Drying the chicken thoroughly ensures it browns rather than steams in the skillet.

2. Sear the Chicken (6–7 minutes | Medium-High Heat)

Heat 1 tablespoon of olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat until the oil shimmers. Add the chicken in a single layer—work in two batches if necessary so you do not crowd the pan. Sear undisturbed for 2 to 3 minutes until a golden-brown crust forms on the bottom, then stir and cook for another 2 minutes. The chicken does not need to be cooked completely through at this stage; it will finish baking in the oven. Transfer the seared chicken and any accumulated juices to a clean plate.

3. Caramelize the Mushrooms (8 minutes | Medium-High to Medium Heat)

Add the remaining 1 tablespoon of olive oil and 2 tablespoons of unsalted butter to the same skillet over medium-high heat. Once the butter foams, add the sliced cremini mushrooms along with the remaining ¾ teaspoon of kosher salt. Stir occasionally for the first 3 minutes as the mushrooms release their moisture, then reduce the heat to medium and cook for 5 more minutes until the liquid evaporates completely and the edges turn deep golden brown.

4. Build the Aromatics and Roux (3 minutes | Medium Heat)

Keep the burner at medium heat. Add the diced shallots (or yellow onion), fresh thyme, and chopped rosemary directly into the mushrooms. Cook for 2 minutes until softened and fragrant. Stir in the minced garlic and cook for 30 seconds until aromatic, taking care not to let the garlic scorch. Sprinkle the 3 tablespoons of all-purpose flour evenly over the mushroom mixture and stir constantly for 1 minute to coat the vegetables and cook out the raw flour taste.

5. Simmer the Cream Sauce (4–5 minutes | Medium-Low Heat)

Slowly pour in the ½ cup of dry white wine while scraping the bottom of the skillet with a wooden spoon to release all the caramelized browned bits (fond). Let the wine bubble and reduce by half, about 1 to 2 minutes. Gradually whisk in the warmed chicken broth, followed by the heavy cream and Dijon mustard. Lower the heat to medium-low and simmer gently for 3 minutes until the sauce thickens enough to coat the back of a spoon. Remove from the heat and stir in ½ cup of the grated Parmigiano-Reggiano and the remaining ¼ teaspoon of black pepper.

6. Assemble and Bake (25 minutes | 375°F / 190°C)

Return the seared chicken (and its resting juices) to the skillet, folding everything together until the chicken and mushrooms are evenly coated in the sauce. Transfer the entire mixture into your prepared 9×13-inch baking dish, spreading it into an even layer. In a small bowl, toss the panko breadcrumbs with 1 tablespoon of melted butter and the remaining ¼ cup of Parmigiano-Reggiano. Scatter the shredded mozzarella over the casserole, then top evenly with the buttered panko mixture. Bake uncovered at 375°F (190°C) for 22 to 25 minutes, until the edges are bubbling vigorously and the breadcrumb topping is crisp and golden brown. Let the casserole rest for 5 to 10 minutes before garnishing with fresh parsley and serving.

Pro Tips for Best Results

• Do Not Overcook the Chicken on the Stove: Searing is strictly for flavor development and color. Pulling the chicken off the stove when it is roughly 75% cooked guarantees it stays juicy and tender after 25 minutes in the oven.

• Warm Your Broth: Pouring cold liquid into a hot roux can cause clumping. Warming the chicken broth for 45 seconds in the microwave helps it emulsify smoothly into the flour and butter base.

• Customizing Add-Ins: If you want to turn this into a complete one-pan meal, fold in 2 cups of fresh baby spinach right before transferring the mixture to the baking dish, or stir in 8 ounces of cooked egg noodles or wild rice alongside the chicken.

Storage, Shelf Life & Reheating

Properly stored, this casserole holds up remarkably well, making it ideal for meal prep and leftovers:

• Refrigerator Longevity: Allow the casserole to cool completely to room temperature (within 2 hours of baking), then cover tightly with foil or transfer portions into airtight glass containers. It will stay fresh in the refrigerator for 3 to 4 days.

• Freezer Longevity (Unbaked): Assemble the casserole up to the point of adding the breadcrumb topping, wrap tightly in a layer of plastic wrap followed by heavy-duty aluminum foil, and freeze for up to 2 months. Thaw overnight in the refrigerator, add the fresh panko topping, and bake at 375°F (190°C) for 30 to 35 minutes until heated through.

• Freezer Longevity (Baked): Baked leftovers can be frozen in airtight containers for up to 2 months. Thaw overnight in the fridge before reheating.

• Reheating Instructions: For individual portions, microwave on medium power (50–70% intensity) in 45-second intervals, adding a splash of chicken broth or milk if the sauce has thickened too much. For a full dish, cover with foil and reheat in a 350°F (175°C) oven for 15 to 20 minutes, uncovering for the final 5 minutes to re-crisp the topping.

Nutrition Facts

Nutritional values are estimates per serving (based on 6 servings using skinless chicken breast and low-moisture mozzarella):

Nutrient	Amount Per Serving	% Daily Value*
Calories	510 kcal	26%
Total Fat	29 g	37%
Saturated Fat	15 g	75%
Cholesterol	165 mg	55%
Sodium	790 mg	34%
Total Carbohydrates	14 g	5%
Dietary Fiber	2 g	7%
Total Sugars	4 g	—
Protein	46 g	92%