Cook pasta according to package directions, omitting salt and fat. Drain; set aside.
Melt 1 tablespoon margarine in a medium heavy saucepan over medium-high heat. Add onion and garlic; sauté 5 minutes or until tender. Add flour; cook 30 seconds, stirring constantly. Gradually add milk; cook 4 minutes or until bubbly. Remove from heat. Stir in 1/4 cup cheese, Italian seasoning, and pepper; remove from heat.
Combine pasta, cheese sauce, 1 cup cheese, tomatoes, and spinach in a large bowl; stir well. Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray.
Combine breadcrumbs, 2 tablespoons cheese, and remaining 1 tablespoon margarine; sprinkle over pasta mixture. Bake at 350° for 30 minutes or until thoroughly heated. Garnish with parsley, if desired.