Cube lean lamb into bite-sized pieces.

Coarsely chop the large onion.

Slice the mushrooms.

Heat butter or margarine in a heavy casserole dish over medium heat.

Add lamb to the casserole and brown on all sides.

Add chopped onion and sliced mushrooms to the casserole.

Sauté until the onion is translucent and the mushrooms are softened.

Season with salt and black pepper.

Pour in the canned tomatoes with their liquid.

Add beef or chicken broth.

Bring the stew to a simmer.

Reduce heat to low, cover, and simmer for approximately 1 hour, or until the lamb is tender.

Stir in whipping cream.

Add dry sherry.

Simmer for another 5 minutes to allow the flavors to meld.

Serve hot.