Make the caramelized onions: Melt 2 tablespoons of butter in a large skillet over medium heat. Add the sliced onions, sugar, and salt.

Cook, stirring occasionally, for 30-40 minutes until the onions are deep golden brown and jammy. If they start to stick, add a splash of water.

Deglaze the pan with beef broth or wine, scraping up all the browned bits. Stir in the fresh thyme and cook until the liquid has evaporated. Set aside to cool slightly.

Spread a thin layer of softened butter on one side of each bread slice.

On the unbuttered side of two slices, spread a tiny bit of Dijon mustard if using.

Layer a generous amount of shredded Gruyère and Swiss cheese onto the unbuttered side of the bread.

Top the cheese with a thick layer of the warm caramelized onions, then add another layer of cheese on top of the onions.

Place the remaining bread slices on top, buttered side facing out.

Heat a clean skillet or griddle over medium-low heat. Place the sandwiches in the pan.

Cook for 3-5 minutes per side, pressing down gently with a spatula, until the bread is golden brown and the cheese is completely melted.

Remove from the pan and let the sandwiches rest for 1 minute before slicing.

Cut diagonally and serve immediately while the cheese is gooey and the onions are sweet and savory.

Pairs perfectly with a side of beef au jus or a simple green salad.