Russian Beef Stroganoff

3 Tbsp. butter
2 large yellow onions, thinly sliced
salt and pepper to taste
1 lb. mushrooms
2 1/2 to 3 lbs. beef chuck roast, cut into 1 in. chunks
2 Tbsp. Dijon mustard
1 c. beef stock
1 c. sour cream

Melt the butter in a large deep skillet over medium-high heat. Add the onions and mushrooms. Sprinkle with salt and pepper. Sir occasionally, until the onions are very soft, about 10 minutes. Add the beef and cook until the outer is no longer pink. Add the beef stock. Adjust the heat so the mixture simmers steadily and cook for about an hour or until the beef is tender. Stir in the Dijon mustard and sour cream. Serve alongside white rice or noodles.

(I served it alongside mashed potatoes; a true Russian Beef Stroganoff isn't served with potatoes I was told)

This can also be made with Beef Tenderloin. Decrease the cooking time for the meat to 10 minutes.