Preheat an outdoor grill for medium/medium high heat.
Fold an 18-inch-wide piece of aluminum foil so it is square.
Place onions, mushrooms, and garlic in the middle of the foil. Arrange butter pieces on top of vegetables and drizzle with steak seasoning. Bring 4 corners of the foil together in the center and crimp any open spaces to seal packet.
Place the foil packet on the preheated grill and cook until vegetables are tender, about 30 minutes.
Remove from grill, open packet carefully, and serve.
