From Healthy Foods: An Irreverent Guide to Understanding Nutrition and Feeding Your Family Well (Champion Press)
Boil water to make boxed mac and cheese. Cook pasta and drain.
In the meatime, chop your onion, open and drain your tuna and get ready to launch into tuna casserole land.
In a medium-sized skillet, melt your smaller amount of butter. Saute your onion and as it turns clear, add the drained tuna and chop it up well in the pan and let it cook for a wee bit. Now add the milk, butter and sauce packet from the box and cook until thickened. Schlep everything together except the saltines and cheddar and stick it in a lightly greased 2 quart casserole dish. Top with the saltines first, then the cheese. Bake for about 15 minutes, give or take, in a preheated 350 degree oven.