Preheat the oven to 350 degrees F (175 degrees C).

Melt butter in a large skillet. Add onions and apples; sauté until tender. Stir in curry powder; sauté 1 minute. Stir in flour; cook 1 minute. Stir in stock and milk until combined.

Meanwhile, season chicken breasts with salt and black pepper; arrange in a single layer in a 9x13-inch (or larger) baking dish. Pour curry sauce over chicken.

Bake in the preheated oven until chicken cooked through, 45 to 50 minutes.