Mushroom Stuffed Burgers

1 1/2 pounds hamburger
1/4 cup finely diced onion
2 eggs, lightly beaten
3/4 cup soft bread crumbs
1/4 cup ketchup
1/2 teaspoon salt
1/8 teaspoon pepper
2 tablespoons butter
1/2 pound sliced mushrooms
6 slices Swiss cheese

Mix well the first 7 ingredients and form into twelve 1/4 inch thick patties. Melt the butter and saute the mushrooms lightly in it. Place on 6 of the patties. Top with remaining patties and seal edges.

Broil or grill to desired doneness. Top with cheese. Serve on toasted buns or as an entree.

Serves 6.