In a small bowl, stir together the red onion, thyme, mayonnaise and mustard. Spread some of this mixture onto one side of each slice of bread. Spread butter onto the other side of the slices of bread.
Heat a large skillet over medium heat. Place 4 slices of the bread into the skillet with the butter side down. On each slice of bread, layer 1/4 of the sliced turkey, then 2 slices of tomato, and top with 2 slices of Swiss cheese. Place remaining slices of bread over the top with the butter side up. When the bottoms of the sandwiches are golden brown, flip over, and cook until golden on the other side.