1In large skillet, heat oil over medium-high heat. Cook mushrooms in oil 3 to 5 minutes, stirring occasionally, until they begin to brown. Add onion; cook 2 to 3 minutes longer. Stir in grill seasoning.
2Set oven control to broil. Line cookie sheet with foil or cooking parchment paper.
3Place bun bottoms on cookie sheet; top each with mushroom mixture and half slice of cheese. Broil 30 to 60 seconds or until cheese is melted. Sprinkle with chives. Cover with bun tops.