Combine breadcrumbs and cheddar cheese; stir breadcrumb mixture well, and set aside.
Steam the broccoli and cauliflower, covered, 4 minutes or until vegetables are crisp-tender. Drain vegetables, and place in a 1 1/2-quart casserole coated with vegetable cooking spray. Combine the margarine, mustard, and white pepper in a small bowl; drizzle over broccoli and cauliflower, and toss well. Sprinkle vegetables with the breadcrumb mixture, and bake at 425° for 8 minutes or until thoroughly heated.