This soup was inspired by licurdia, a Calabrian staple starring the region’s sweet, shallot-shaped Tropea onions. We found that typical American sweet onions, like Vidalia, were too one-note, but a combination of sharp red onions and milder yellow worked well. As the onions cook down, a fond forms on the bottom of the pot. This gives the broth color and flavor, so allow it to take on a deep, rich caramelization. Like French onion soup, this Italian iteration likes cheesy bread. Tangy Caciocavallo cheese from Southern Italy is ideal. If you can’t find it, aged provolone or smoked mozzarella are excellent, too.