Partially cook the macaroni in salted water and drain well. Put in a baking dish and pour half of the butter over the macaroni. Beat the egg whites and mix in the macaroni. Keep the yolks.They will be used to make the bechamel sauce. Sprinkle with half of the cheese. With the remaining butter brown the minced meat and the onion. Add the strained tomatoes or the tomato paste, salt, pepper and simmer until liquid is absorbed. Remove from the heat and add the remaining grated cheese. Stir all well. Prepare the béchamel sauce. Butter a baking dish. Lay half of the macaroni in the pan. Spread evenly with the meat mixture. Lay the rest of the macaroni on top. Pour the bechamel sauce over them and sprinkle with grated cheese and some melted butter. Bake in a moderate oven (200 C) until bechamel gets brown (40 - 45 minutes).