1 Cook macaroni in a kettle, following label directions; drain; return to kettle.
2 While macaroni cooks, melt butter or marga-rine in a medium-size saucepan; remove from heat.
Toss 1 tablespoon with bread crumbs and Parmesan cheese in a small bowl; set aside for Step 4.
3 Stir flour, salt, mustard, and pepper into remaining butter or margarine in saucepan; cook, stirring all the time, just until mixture bubbles.
Stir in milk slowly; continue cooking and stirring until sauce thickens and boils for 1 minute; stir in Cheddar cheese until melted.
4 Stir corn, tuna, mushrooms and liquid, and cheese sauce into drained macaroni in kettle.
Spoon into a 10-cup baking dish; sprinkle with bread crumb-cheese mixture.
Cover and chill.
5 One hour before serving time, uncover casserole; place in a cold oven; set heat control at moderate (350°).
Bake for 1 hour, or until bubbly hot in center and crumbs are toasted.