Preheat oven to 350°F. Boil water to make macaroni and cheese. Cook pasta and drain. In the meantime, chop your onion, open and drain your tuna and get ready to launch into tuna casserole land. In a medium-sized skillet, melt 1/2 tablespoon of butter. Sauté your onion and as it turns clear, add drained tuna. Then add the milk, butter and sauce packet from the box of macaroni and cheese and cook till it thickens. Mix everything together except the saltines and cheddar, and place it in a lightly greased casserole dish. Top with saltines and then cheese. Bake for about 15 minutes. Serves 4.