Caramelized Onion and Gruyere Skillet Tarts

In the realm of favorite food smells, it's hard to top the aroma of onions caramelizing in a pan. Not only does it bring me right back to childhood, because my mother believed sauteed onions made everything better, but it makes me instantly hungry. All onions are capable of doing this but it would be such a shame to pass up the sweet summer onions while we have them. I want to caramelize them