Line a big enough baking dish for your roast with foil, with enough extra to fold over and seal in roast and ingredients.
Place roast in middle of pan, sprinkle the onion soup mix evenly over top and sides of roast, rub in a little. Spoon mushroom soup over the roast. Cover the roast in the foil and pinch sides to seal everything in. Cook in 350 oven to desired doneness.
When done carefully open foil (hot and steamy), remove roast to carving board allow to rest (while you mash the potatoes). Pour gravy from foil into gravy boat, stir. Slice and serve with mashed potatoes, the self-made gravy, and a veggie.
Here's what I do with the left overs from this: Prepare 1-2 packets of Lawry's Au Jus mix according to directions. To the pot add your left over gravy and sliced roast beast. Allow to simmer on low for 20 minutes to an hour, or until meat is tender. Serve on hoagy rolls with a bowl of the au jus for dipping. Viola! French Dip Sandwiches. Serve with tater tots and a salad, and the gang will love you.