Slice the yellow onions thinly and heat a skillet over medium-low heat. Add olive oil and butter, then introduce the onions. Cook for 30–45 minutes, stirring occasionally, until they turn a golden brown and soft. Once caramelized, add brown sugar and balsamic vinegar; season with salt and pepper before removing from heat. Set these delicious onions aside for later.

In the same skillet, increase the heat to medium and add a splash of olive oil. Toss in the sliced cremini mushrooms and sauté for 8–10 minutes until they are browned and tender. Add minced garlic and fresh thyme leaves, cooking for another minute until fragrant. De-glaze the pan with white wine, allowing it to reduce slightly before seasoning with salt and pepper, then take off the heat.

While your mushroom mixture cools, take slices of sourdough bread and spread softened butter generously on one side of each slice. Spread Dijon mustard on the unbuttered side of two of the slices.

Layer Swiss cheese on the unbuttered side of one slice of bread, followed by a generous helping of caramelized onions and sautéed mushrooms. Top with another slice of bread, buttered side out.

Heat a skillet over medium heat and carefully place the sandwiches in the pan. Grill each side for about 3–4 minutes or until the bread is golden brown and crispy, and the cheese melts beautifully.

Once grilled to perfection, remove the sandwiches from the skillet and slice them in half. Serve warm.