In a large cast-iron or other heavy skillet, saute onions in 1 tablespoon butter until tender; remove and set aside. Spread barbecue sauce on 4 slices of bread. Layer each with 1 slice of cheese, 2 slices each of bacon and turkey, pickle slices, onions and another slice of cheese. Cover with remaining slices of bread. , In the same skillet over medium-low heat, melt remaining butter. Cook sandwiches on both sides until golden brown and cheese is melted. Cover the skillet during the last few minutes to help melt cheese if necessary.