Cook macaroni in lightly salted boiling water about 5-7 minute or until macaroni is aldente.
While macaroni is cooking, beat egg with salt, mustard in pepper in a medium bowl. Stir in cottage cheese, sour cream, milk and 1/2 of cheddar cheese. Save remaining cheese to sprinkle on top. Refrigerate.
Drain macaroni well and rinse with hot water. Place into 2 quart casserole and keep warm.
Stir egg mixture into cooked macaroni. Break tuna into chunks, carefully stir tuna into macaroni mixture. Sprinkle cheese on top. Cover with foil.
Bake in preheated oven 45 minute Remove cover and bake 10 minute longer or until lightly browned. Serve hot in casserole or spoon onto 6 plates.
This recipe is good doubled and can be frozen and re-heated in the microwave!