Begin by slicing your yellow onions thinly. In a large skillet over medium-low heat, combine 1 tablespoon each of olive oil and butter. Add the onions, stirring occasionally for 30-45 minutes until they turn golden brown and soft. Enhance the flavor by adding brown sugar and balsamic vinegar, cooking for an additional 5 minutes until glossy. Season with salt and pepper to taste.

Using the same skillet, add another tablespoon of olive oil and raise the heat to medium. Toss in sliced cremini mushrooms, sautéing them for 8-10 minutes until they’re beautifully browned and tender. Next, add minced garlic and fresh thyme, stirring until fragrant. If desired, de-glaze the pan with a splash of white wine, scraping the bottom for flavor. Season the mushroom mixture with salt and pepper.

While your mushrooms are sautéing, take your slices of sourdough bread and butter one side of each. On the unbuttered side of two slices, spread a thin layer of Dijon mustard for added flavor.

On the mustard-spread side of each piece of bread, layer a generous slice of Swiss cheese, followed by a spoonful of your caramelized onions and a layer of sautéed mushrooms. Top with another slice of Swiss cheese before placing the second piece of bread on top, butter side facing out.

Heat a skillet over medium heat and carefully place the assembled Swiss Mushroom Melt Sandwiches in the pan. Grill each sandwich for 3-4 minutes on each side or until the bread is golden brown and the cheese is deliciously gooey.

Once your Swiss Mushroom Melt Sandwiches are perfectly grilled, remove them from the skillet and slice them in half. Serve warm, ideally paired with a side salad or comforting tomato soup.