Form the ground sirloin into 4 (8-ounce) patties. Over an open-flame, grill each burger to desired temperature. While the patties are grilling, cook bacon until desired crispness, and set aside.
Butter both sides of each bun and grill until toasted. On each of the 4 bottom buns, place the burger, followed by the blue cheese spread, mushrooms, and bacon, in that order. On each of the top 4 buns, place the lettuce, tomato and red onion, in that order. Top with any additional condiments desired, sandwich the 2 sides, and serve.
Blue Cheese Spread:
1 1/4 cups crumbled blue cheese
3/4 cup sour cream
Combine both ingredients in a bowl and refrigerate for 2 hours prior to use.
1 tablespoon butter
1 tablespoon minced garlic
8 ounces sliced mushrooms
2 ounces white wine
Melt butter in a large saute pan over medium heat. Add garlic and mushrooms and saute until tender. Add wine and simmer for 1 minute or until almost evaporated. Remove from heat and set aside until ready to use.