Cook macaroni according to package directions; drain.
In saucepan, melt 6 tbsp butter or margarine over low heat.
Blend in flour and 3/4 tsp salt. Add milk all at once.
Cook quickly, stirring constantly, until mixture thickens and bubbles.
Add cheese; stir until cheese is melted.
Stir in tuna, diced apples and drained macaroni; turn into 12x7x2 inch baking dish.
Combine melted butter and crumbs. Sprinkle on top of casserole.
Bake in 350° F oven for 30 minutes, or until apples are tender.
Servings: 8