For 4 - 5 burgers:
6 large white mushrooms, chopped
1/2 large onion, chopped
1 1/2 pounds lean ground sirloin
Saute mushrooms and onions in butter until tender. Season with salt and pepper. Place sirloin into a large bowl and mix in mushroom mixture. Form good-sized patties. Grill until medium-well, or dip in flour and pan fry in a little olive oil. Serve on lightly toasted buns with a dollop of sour cream.