Melt butter in a medium skillet over medium heat. Add onion and garlic until onions are translucent. Stir in mushrooms and sauté until soft. Transfer to a medium bowl and allow to cool. Add ground venison, blue cheese, breadcrumbs, salt and pepper and mix well with your hands to blend. Form into 4 large patties. Grill, pan-fry or broil patties until browned. Add to bun with lettuce, tomato and your choice of other condiments.