- Melt half of the butter in nonstick skillet set over medium heat; cook onion, sugar, vinegar, Worcestershire sauce, thyme and rosemary, stirring occasionally, for about 20 minutes or until onion is soft and caramelized.
- Let cool to room temperature; fold in Gruyère.
- Spread mustard evenly over 2 slices of bread.
- Top with onion mixture; top with remaining bread.
- Spread remaining butter over outside of sandwiches.
- In nonstick skillet set over medium-low heat, cook sandwiches for 5 to 7 minutes per side or until cheese is melted and bread is golden brown.
Tip: Add dried cherries to onion mixture for a sweet note.