Start by thinly slicing the yellow onions. In a large skillet, heat 1 tablespoon of olive oil and 1 tablespoon of butter over medium-low heat. Add the onions and sauté for 30–45 minutes until deep golden brown.

Once the onions are caramelized, sprinkle in 1 teaspoon of brown sugar and drizzle 1 tablespoon of balsamic vinegar. Stir well and cook for an additional 2–3 minutes, then set aside.

In the same skillet, add another tablespoon of olive oil and raise the heat to medium. Sauté the sliced cremini mushrooms for 8–10 minutes until nicely browned.

Add 2 minced garlic cloves and 1 teaspoon of fresh thyme to the skillet. Cook for 1–2 minutes until fragrant.

Pour in 1/4 cup of dry white wine to deglaze the skillet. Let it simmer for 3–4 minutes until most of the liquid has evaporated.

Spread butter on one side of each slice of bread and a thin layer of Dijon mustard on the unbuttered sides of two slices.

Layer a slice of Swiss cheese on the mustard-side of each bread slice, followed by sautéed mushrooms and caramelized onions. Top with another layer of Swiss cheese.

Heat a skillet over medium heat and place the assembled sandwiches in the pan. Cook for 3–5 minutes on each side using a lid to help the cheese melt.

Remove from skillet and allow to cool slightly before slicing in half. Enjoy immediately.