Coat a 2-quart oval or rectangular baking dish with cooking spray; set aside. In a large saucepan cook potatoes, covered, in a moderate amount of boiling salted water about 5 minutes or just until tender. Drain and transfer to an extra-large bowl.
For sauce, in same saucepan melt 2 tablespoons of the margarine or butter over medium heat; add onion and garlic and cook about 5 minutes or just until tender. Stir in flour, rosemary, parsley, salt, and black pepper. Stir in milk all at once. Cook and stir over medium heat until thickened and bubbly.
Add spinach or broccoli rabe and sweet pepper to potatoes. Toss gently to combine. Pour sauce over potato mixture. Stir gently until coated. Spoon potato mixture into prepared baking dish (dish will be very full). Set baking dish on a baking sheet.
Sprinkle with cheese. Melt the remaining margarine or butter. Add bread crumbs, tossing to coat. Sprinkle over cheese. Bake in a 375 degree F oven about 20 minutes or until edges are bubbly and crumbs are golden brown. Makes 10 to 12 servings.