Heat a skillet over medium heat and add a tablespoon of olive oil or butter. Once melted, add thinly sliced onions and stir gently to coat. Cook for 15-20 minutes, stirring occasionally, until the onions are golden brown and soft.

While the onions are caramelizing, gather two generous slices of hearty bread. Layer a handful of shredded Gruyère or Swiss cheese on one slice, add caramelized onions, and finish with another layer of cheese before placing the second slice of bread on top.

In the same skillet, add another tablespoon of butter or olive oil and increase heat to medium-high. Carefully place the assembled sandwich in the skillet and grill for about 3-5 minutes until golden.

After flipping, cook for an additional 3-5 minutes until the other side is golden brown and the cheese is melted.

Transfer the sandwich to a cutting board, let it sit for a minute, then slice and serve with tomato soup or salad.