Roast beef + Onion soup mix + some root veg (potatoes, carrots, turnips, parsnips, whatever you like) + water + time = pretty yummy. And if you are a stickler you can season the roast and sear it beforehand.

Though sometimes the sauce is kinda thin so I will pour it out into a pan and reduce it and thicken it either by the reducing it or by a slurry of cornstarch!