Combine bison, eggs, Worcestershire sauce and Dijon mustard, black pepper, beef base, bread crumbs and blend well by hand. Roll into a log, slice into 5 equal portions and shape each into a patty. Grill to medium well.
As patties cook, sauté sliced mushrooms (with olive oil, salt, pepper and pinches of dry herbs). Remove patties to a baking sheet, cover with sautéed mushrooms and a thick slice of Havarti cheese, melt under broiler to melt slightly.
Place cooked bison patties on fresh rolls and serve with tomato, onion and lettuce.
