Begin by thinly slicing the yellow onions and adding them to a skillet over medium-low heat. Sauté with olive oil and butter for 30–45 minutes until golden brown.

Once the onions are caramelized, sprinkle in brown sugar and balsamic vinegar, and cook for an additional 2–3 minutes. Set aside.

In the same skillet, add more olive oil and sauté the sliced cremini mushrooms for 8–10 minutes until browned.

Add minced garlic and thyme to mushrooms, cooking for an additional minute. Deglaze the pan with white wine.

Butter one side of each slice of sourdough bread. Spread Dijon mustard on unbuttered sides of two slices.

Layer Swiss cheese on the mustard-coated side, followed by caramelized onions and sautéed mushrooms. Repeat layers.

Grill the sandwich in the skillet over medium heat for 3–5 minutes per side until cheese melts and bread is golden.

Remove from skillet, let rest for a minute, then cut in half and serve warm.