Preheat oven to 350°F (175°C).

Cook mostaccioli according to package directions, omitting salt and fat. Drain and set aside.

Melt 1 tablespoon of reduced-calorie stick margarine in a medium heavy saucepan over medium-high heat.

Add vertically sliced onion and minced garlic and sauté for 5 minutes, or until tender.

Add all-purpose flour and cook for 30 seconds, stirring constantly.

Gradually add skim milk, cook for 4 minutes or until bubbly.

Remove from heat.

Stir in 1/4 cup of preshredded Parmesan cheese, dried Italian seasoning, and pepper; remove from heat.

Combine cooked pasta, cheese sauce, 1 cup of preshredded Parmesan cheese, diced tomatoes with basil, garlic, and oregano, and thawed and drained frozen chopped spinach in a large bowl.

Stir well to combine.

Spoon the mixture into a 13 x 9-inch baking dish coated with cooking spray.

Combine dry breadcrumbs, 2 tablespoons of preshredded Parmesan cheese, and remaining 1 tablespoon margarine in a small bowl.

Sprinkle the breadcrumb mixture evenly over the pasta mixture in the baking dish.

Bake at 350°F (175°C) for 30 minutes, or until thoroughly heated and bubbly.

Garnish with parsley sprigs, if desired, before serving.