Cook manicotti shells according to package directions. Drain; rinse with cold water. Drain well. Cook sausage and garlic in a large saucepan until cooked through, breaking up sausage with a spoon. Remove from heat. Add mozzarella cheese, spinach, ricotta cheese, the 1/4 cup grated Parmesan cheese, Italian seasoning, lemon juice, and egg; mix well. Carefully spoon mixture into cooked shells.
Cut each manicotti diagonally crosswise into three pieces. Place in a 3-quart rectangular baking dish. Spoon spaghetti sauce over. Bake, covered, in a 350 degree F oven for 30 to 35 minutes or until heated through. If desired, sprinkle with 2 tablespoons shredded Parmesan cheese. Makes 15 servings.