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 45 minutes. Remove from grill, open packet carefully, and serve.