Creating your own French Chicken Casserole à la Normande can be a straightforward and enjoyable process. Follow these steps to make this marvelous dish:

1. Preheat Oven: Preheat your oven to 350°F (175°C).
2. Sauté Vegetables: In a large skillet, melt 1 tablespoon of butter over medium heat. Add the onion and garlic. Sauté until soft and fragrant, about 3 minutes.
3. Cook Chicken: In the same skillet, add the chicken breasts, cooking until browned on both sides, approximately 5-7 minutes. Remove the chicken and set aside.
4. Add Remaining Vegetables: In the skillet, add the carrots, celery, and mushrooms. Cook for another 5-6 minutes until they start to soften.
5. Make the Sauce: Pour the chicken stock into the skillet. Stir in the heavy cream, thyme, salt, and pepper. Bring to a slight simmer, then add the Dijon mustard for an extra punch.
6. Combine Ingredients: Place the chicken back into the skillet, ensuring it’s coated with the sauce.
7. Transfer to Casserole: Carefully transfer the mixture to a casserole dish, and spread it out evenly.
8. Bake: Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 20 minutes to allow the top to brown slightly.
9. Finish and Garnish: Remove the casserole from the oven and allow it to cool for about 10 minutes. Garnish with fresh parsley before serving.