Put your onions in a medium/large skillet pan over medium heat.  Cover and cook, only stirring occasionally, about 15 minutes. Add the oil and a pinch of salt and reduce the heat to medium low.  Continue to cook, stirring occasionally, until the onions are caramelized to your liking.  It can take 5 minutes or another 45 minutes, depending on how deeply caramelized you like them.

(Tip:  if you like caramelized onions a lot, double or triple this batch so you have some extra to keep in the refrigerator.  They taste great on everything!)

When your onions are ready, you are ready to make your sandwich.  Start by buttering the outside of your bread pieces and adding a little butter to a skillet over medium heat.  Add enough cheese to cover your bread slice, then add enough tomato slices to also cover the area of the bread.  Top with half of the caramelized onions and the final piece of bread.

Place the sandwiches in the skillet and cook for about 2-3 minutes on one side.  Carefully flip the sandwich and cook another 2-3 minutes.  If your cheese is not melted (sometimes I slice my cheese a little too thick…) add about a tablespoon of water to the skillet, not too close to the sandwich so it gets your bread wet, and cover the skillet. The cheese should melt quickly. Enjoy!