Cook bacon until crisp; drain on paper towels and crumble. Spray a 9x12x2 casserole dish with vegetable spray. Place croutons in casserole and pour margarine over them. Sprinkle grated cheese over all. Mix milk, eggs and mustard and pour over cheese. Sprinkle bacon crumbs over all. Bake at 325° for 45 minutes. Allow casserole to stand for 15 minutes. Serves 8 to 10.