Melt butter in large skillet; add onions and sprinkle with 1 tablespoon seasoning. Sauté onions until tender, about 7 to 10 minutes.
Combine ground sirloin and 3 tablespoons seasoning in a bowl and mix thoroughly. Form seasoned sirloin into evenly sized patties. Place burgers on grill. Grill 7 to 10 minutes or until internal temperature reaches a minimum of 165ºF.
Place grilled burgers in bottom portion of buns. Cover each burger with equal amounts of sauteéd onions and blue cheese. Top with bun and serve.