Start by seasoning the chicken thighs with salt and pepper. Heat olive oil in a pan and sear the chicken until golden brown on both sides. This step locks in flavor and creates a beautiful color. Remove the chicken and set it aside.
In the same pan, add chopped onions and cook until soft and translucent. Stir in garlic and cook briefly until fragrant. This creates the flavorful base of the casserole.
Add sliced apples and mushrooms to the pan and cook until slightly softened. The apples release a subtle sweetness while mushrooms bring depth. Stir everything together for even cooking.
Pour in the cider and let it simmer to reduce slightly. Add chicken stock, Dijon mustard, and thyme, then stir well. This forms a rich, aromatic sauce with balanced flavors.
Return the chicken to the pan and spoon sauce over it. Add cream and butter, then transfer everything to an oven-safe dish if needed. Bake until the chicken is fully cooked and tender.
Once cooked, let the casserole rest for a few minutes. The sauce will thicken slightly as it cools. Serve warm with your favorite sides for a comforting meal.