Start by seasoning the chicken thighs with salt and pepper. Heat butter and olive oil in a skillet over medium heat. Sear the chicken until golden on both sides, then remove and set aside.
In the same pan, add chopped onions and cook until soft and translucent. Stir in garlic and cook briefly until fragrant. This forms the flavorful base of the casserole.
Toss in sliced apples and mushrooms, cooking until slightly softened. The apples release natural sweetness, while mushrooms deepen the savory profile. Stir gently to combine flavors.
Pour in cider and chicken stock, scraping the pan to lift all the flavorful bits. Add Dijon mustard and thyme, then stir in the cream. Let it simmer gently until slightly thickened.
Return the chicken to the pan, coating it well with the sauce. Transfer everything into a baking dish. Ensure the chicken is partially submerged for even cooking.
Place the dish in a preheated oven at 180°C (350°F). Bake for about 25–30 minutes until the chicken is fully cooked and tender. The sauce should be bubbling and creamy.
Once baked, sprinkle fresh parsley over the top. Let it rest for a few minutes before serving. This helps the sauce settle and enhances flavor.