Melt butter in large skillet; add onions and sprinkle with 1 teaspoon seasoning. Sauté onions until tender, about 7 to 10 minutes.
Combine ground sirloin, 4 teaspoons of seasoning, and Worcestershire sauce in a bowl and mix thoroughly. Form seasoned sirloin into 4 evenly sized patties. Place burgers on grill over Direct Medium heat. Grill 12 to 16 minutes, turning once, or until internal temperature reaches a minimum of 165ºF.
Place grilled burgers in bottom portion of buns. Cover each burger with equal amounts of sautéed onions and about 1-1/2 tablespoons blue cheese. Top with bun and serve.