Gruyère with Caramelized Onions

This is one of Tom’s favorites, one that showcases how a sandwich can be adapted from a meal. The meal here is French onion soup. Whereas in the soup the onion is the star, with the crouton and cheese to support it, in the sandwich we flip that: the onions serve as the relish. We roast the onions very slowly, caramelizing them, until dark golden brown to balance the assertive and pungent Gruyère. With the rye bread to hint at the Alsatian origin of the soup, we end with something rustic and homey, and yet something that, unlike soup, can be enjoyed while walking down the street.