Preheat oven to 375 degrees. Grease 2 quart casserole dish or 11x9 inch pan.
Add cornstarch, slat, mustard and pepper in a medium saucepan. Stir in evaporated milk, water and margarine. Cook over medium-high heat, until sauce comes to a boil. Boil for 1 minute. Remove from heat.
Stir in 1 1/2 cups cheese until melted. Add macaroni; mix well. Pour into casserole dish, top with remaining cheese and bake 20-25 minutes until light brown on top.