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.
Combine both ingredients in a bowl and refrigerate for 2 hours prior to use.