Preheat oven to 400 degrees F. In large mixing bowl, combine turkey, spinach, diced onion, garlic, breadcrumbs, Parmesan, eggs, milk, Italian seasoning and cayenne pepper. Mix until all ingredients are combined; then mix in 1-1/4 cups Mozzarella cheese.
Line rimmed baking pan with foil and spray with cooking spray. Form mixture into 1-1/2 inch balls and place on baking pan. Bake 25 minutes, or until meatballs reach 180°F.
While meatballs are baking, heat olive oil in large heavy-bottomed pot. Cook bell peppers and sliced onion until they begin to soften, about 5 minutes. Add pasta sauce and heat through. When meatballs are done cooking, add to sauce and simmer 10 minutes.
Split buns and fill with meatballs and sauce. Top with remaining Mozzarella.