Preheat oven to 400F. Spray a shallow 1 1/2-qt casserole with nonstick spray; set aside. Spray a 10" skillet with nonstick spray; heat. Add onion; cook over medium heat, stirring constantly with a wooden spoon, until translucent, 2-3 minutes.
Add turkey; cook, stirring constantly to break up meat, until browned. Add pasta and sauce; mix well.
Transfer mixture to prepared casserole, spreading evenly. Drop teaspoonfuls of ricotta cheese evenly over pasta mixture. Bake until mixture is heated and ricotta cheese melts, 10-12 minutes. Remove from oven; sprinkle evenly with mozzarella cheese. Bake until mozzarella is melted and mixture is steaming hot, 10-12 minutes longer.