Preheat your oven to 375°F (190°C). Season the chicken breasts with salt, pepper, and a pinch of thyme. Heat olive oil in a skillet over medium-high heat, then sear the chicken for about 3-4 minutes on each side until golden brown. Remove and set aside.
In the same skillet, add sliced onions and cook until they soften and turn golden, about 7-8 minutes. Stir in minced garlic and cook for another minute until fragrant. Transfer the onions and garlic to your baking dish.
Add sliced apples to the skillet and cook until they start to caramelize, about 5 minutes. Spread the apples evenly over the onions in the baking dish.
Place the seared chicken breasts on top of the apple and onion mixture. Pour heavy cream over the chicken, then sprinkle with chopped thyme, salt, and pepper to taste.
Sprinkle breadcrumbs evenly over the top of the casserole, which will create a crispy crust as it bakes. Cover with foil if desired to prevent burning.
Bake in the preheated oven for about 30-40 minutes, until the chicken is cooked through (internal temperature of 165°F) and the top is golden and bubbly. Remove from the oven and let rest for a few minutes.
Serve hot, scooping generous portions of the creamy sauce, caramelized apples, and tender chicken, with the crispy breadcrumb topping on top. Enjoy the warm, fall-inspired flavors!