El Yucateco Black Reserve Habanero Hot Sauceadjust amount to taste
Heat skillet over medium heat. Add oil. When the oil is hot, add the sliced onions and cook until caramelized, about 20 minutes.
Meanwhile, butter one side of each slice of bread. This will be the side to go on the outside of the sandwich.
Place a slice of gouda cheese on top of the unbuttered side of the bread. then add a tomato slice and then about a tablespoon of caramelized onions. Then top with another slice of cheese. Lastly top with the other slice of bread (buttered side up).
Place the sandwich (buttered side down) in the skillet. Let cook about 5 minutes or until the cheese is melted and the bread is golden brown.
Using a spatula, flip the sandwich so the other side of the sandwich is facing down on the skillet. Let cook for a few minutes until the cheese on the other side of the sandwich is melted, and the bread is golden brown.