The original recipe I found used ground beef and an onion soup mix, but I've adapted it, added some things, and made it yummier and healthier to boot!
1 c Leftover beef (from steak, roast, etc) chopped into bite-sized pieces
1 chopped onion
3/4-1 c sour cream
1-2 cans cream of mushroom soup
4 Tbs Worcestershire sauce
4 c cooked pasta
(amounts are pretty flexible, I just put approximately what I use--I don't measure!)
Set noodles to cook (they will need around 15 min, and everything else will cook within that time)
If beef is not pre-cooked, then put beef and onion in a hot skillet and saute until meat is cooked and onions look glassy (to prevent sticking, add a little oil if beef is not fatty). If beef IS pre-cooked, then just saute onions, and add beef after onions are glassy. Mix in Worcestershire as the beef cooks.
Dump in sour cream and cream of mushroom soup with the meat/onions, and stir in pan until mixed and warmed-through.
Serve over noodles!