Pat the chicken breasts dry and season with salt and pepper. In a large skillet, melt butter over medium-high heat and sear the chicken for 6-8 minutes on each side until golden brown and cooked through. Remove the chicken and cover to keep warm.

In the same skillet, add more butter if needed and let it melt. Sauté chopped onions and celery for 4-5 minutes until softened. Add minced garlic and cook for an additional minute.

Add more butter to the skillet and heat. Add sliced mushrooms and let them cook undisturbed for 3-4 minutes until browned.

Return the sautéed onions and celery to the skillet with the mushrooms and add wild rice. Stir and cook for 2 minutes. Pour in chicken broth and white wine, bring to a boil, then reduce heat and simmer for 15-20 minutes until the rice is tender.

Melt remaining butter in a small saucepan over medium heat. Whisk in flour and cook for 1-2 minutes. Gradually add milk while stirring until the mixture thickens.

Once the rice mixture is cooked, gently fold in the white sauce and nestle in the seared chicken. Preheat oven to 400°F and bake uncovered for 20-25 minutes until bubbly.

Let the casserole rest for 10 minutes before serving. Enjoy!