Line baking pan with parchment paper. Preheat oven to 400 degrees Cut spaghetti squash in half, seed, then slice in 1 pieces. Rub with olive oil. Place on sheet pan. Place chicken breasts on sheet pan, scatter chopped onions and tomatoes throughout. Cover with pasta sauce. Sprinkle Italian seasoning over all ingredients. Bake for 15 minutes. Remove sheet pan to add mozzarella cheese to chicken breasts, then cook for 5 more minutes or until chicken is thoroughly cooked.