Preheat the oven to 400 degrees F (200 degrees C). Spray the top and bottom parts of a broiler pan with cooking spray.

Melt butter in a skillet over medium heat. Add onion and season with salt. Sauté for 2 minutes. Add sugar and continue cooking until onion is translucent but not browned, 6 to 8 minutes. Remove from heat.

Layer apples, potatoes, and sautéed onion in the bottom part of the broiler pan. Cover with the part top of the broiler pan.

Drizzle olive oil over chicken thighs. Sprinkle with rosemary, salt, and pepper. Rub thoroughly over each thigh and place on the top part of the broiler pan.

Roast in the preheated oven until juices run clear and an instant-read thermometer inserted into the thickest part of the thigh, near the bone, reads 165 degrees F (74 degrees C), about 1 hour. Serve apples, potatoes, and onion alongside chicken thighs.