If you go with a full sized brisket, add two additional cans of cream of mushroom soup and one more envelope of soup mix. Cut brisket in half and stack the halves (with a can of soup and half envelope of soup mix between them.
Take two long sheets of aluminum foil, lay them on top of each other and fold al the way across the long side, then repeat. Open the sheets- you should have "one" very large and wide sheet.
Scoop out one can of soup, top with half the soup mix.
Place brisket, fat side up, on top of the soup mixture. Add remaining cans of soup and onion mix on top of the meat, stir together (does not have to be completely or evenly mixed).
Bring the sides of the foil up and fold together.
Place foil pouch inside a large lidded pot. Cook in preheated oven for 3 hours. Serve with mashed potatoes.
Notes
Can also be done in a slow cooker- LOW for 8 hours, remove from insert. Whisk in one packet brown gravy mix and cook on HIGH for 20 minutes, or until thickened.