Start by preheating your oven to 350°F (175°C). This ensures it’s ready when the casserole goes in. 🔥
Melt half the butter in a large skillet over medium heat. Sauté the sliced mushrooms until golden and tender, then remove them from the skillet. This step adds a rich, savory flavor to the casserole. 🍄
Add the remaining butter to the skillet along with the diced celery and carrots. Cook for about 10 minutes, stirring occasionally, until they’re soft and slightly caramelized. This creates a flavorful base for your creamy sauce. 🥕
Stir in the flour, cooking for two minutes to form a roux. Slowly whisk in the chicken broth and whole milk, making sure there are no lumps. Simmer for 5-7 minutes until the sauce thickens.
Add the diced chicken, sautéed mushrooms, frozen peas, thyme leaves, salt, and pepper to the sauce. Stir everything together, ensuring the flavors meld beautifully. 🍗
Mix breadcrumbs, Parmesan cheese, parsley, and melted butter in a bowl. The topping should have a buttery, crumbly texture, perfect for baking.
Pour the chicken and vegetable mixture into a casserole dish. Spread the breadcrumb topping evenly across the top to create that delicious crunch.
Transfer the casserole dish to the preheated oven. Bake for 30-35 minutes, or until the topping is golden brown and the filling is bubbling around the edges.
Once out of the oven, allow the casserole to cool for a few minutes. Garnish with fresh parsley for a burst of color and serve! 🌿