Slice the yellow onions thinly and place them in a skillet over low heat. Add olive oil and butter, allowing the mixture to melt together. Stir occasionally for 30–45 minutes until the onions achieve a deep golden brown color and develop a sweet aroma.

Once the onions are caramelized, sprinkle in the brown sugar and drizzle the balsamic vinegar over them. Cook for an additional 2-3 minutes, stirring frequently until the mixture thickens and is well absorbed. Season with salt and pepper to enhance the flavors.

In the same skillet, add a splash of olive oil and toss in the cleaned, sliced cremini mushrooms. Sauté over medium heat for 8–10 minutes until they are browned and tender, stirring occasionally for even cooking.

Stir in the minced garlic and fresh thyme leaves to the sautéed mushrooms. Cook for about 1-2 minutes, or until the garlic becomes fragrant and golden. Pour in the dry white wine, scraping the bottom of the skillet to deglaze it, and let it simmer until most liquid evaporates.

Butter one side of each slice of sourdough bread. On the unbuttered side of two slices, spread a thin layer of Dijon mustard.

Layer half of the Swiss cheese on each slice of bread. On top of one cheese layer, add a generous portion of the caramelized onions, followed by the sautéed mushrooms, then repeat with another layer of cheese and the second slice of bread.

Heat the skillet over medium-low heat. Place the assembled sandwiches into the skillet and cover with a lid. Cook for about 3–5 minutes on each side, or until the bread is golden brown and the cheese has melted beautifully.

Once both sides are crispy and the cheese is thoroughly melted, transfer the sandwiches to a cutting board. Allow to cool for a minute, then slice them in half to reveal the delicious layers inside. Serve warm with a side of fresh salad or crispy chips.