In a medium skillet melt 1 tablespoon butter over medium-low heat. Add onions. Cook, uncovered, until tender and lightly browned, about 20 minutes, stirring occasionally. Carefully add wine and thyme. Cook, stirring frequently, until most of the liquid has cooked off. Season to taste with salt and pepper. Remove from the heat and set aside.
Place one slice of bread on work surface; top with Swiss cheese and caramelized onions. Top with remaining bread slice. Spread with half the remaining butter. Place, buttered side down, in a skillet over medium heat. Carefully butter top bread slice. Cook for 2 to 3 minutes per side or until cheese is melted.