Melt 1 tablespoon butter in skillet over medium heat. Add onions and sugar to pan. Reduce heat to medium low. Cook onions, stirring occasionally, 15-20 minutes until golden brown.

Increase heat to medium. Add in thyme and cook for about 1 minute. Pour in wine and stir until most of wine has cooked off.

To assemble sandwich, place 1/2 of cheese on one slice of bread, followed by caramelized onions, top with remaining cheese and other slice of bread.

Melt remaining 1 tablespoon of butter on skillet or panini press. Add sandwich and cook until lightly browned on one side and flip or until panini press is ready.