Shred cabbage. Cook 5-8 minutes in boiling water with the salt. Don't overcook. Cabbage should remain crisp. Drain well. Layer cabbage in buttered 2 quart casserole dish: cabbage, enough sauce to cover, second layer of cabbage, remaining sauce, then cheese. Top with buttered crumbs. Bake in preheated oven at 300 degrees F for 15-20 minutes. For White Sauce: Melt margarine in a sauce pot. Add flour, salt, and pepper. Add milk and cook until thick, stirring constantly.