Preheat oven to 375°.
Place bread cubes in a food processor; pulse until finely ground. Set aside.
Steam the cauliflower, covered, 13 minutes or until tender; set aside. Melt 1 tablespoon margarine in a medium nonstick skillet coated with cooking spray over medium-high heat. Add onion, bell peppers, and garlic; sauté 5 minutes or until tender. Combine the onion mixture, steamed cauliflower, 1 tablespoon margarine, water, salt, and white pepper in food processor; process until smooth, scraping sides of processor bowl occasionally.
Combine cauliflower mixture and 1 1/2 cups breadcrumbs, and spoon into a 1-quart casserole dish coated with cooking spray. Sprinkle with 1/2 cup breadcrumbs. Bake at 375° for 15 minutes or until thoroughly heated.