Classic versions of the patty melt often include caramelized onions, but those take time, so here cookbook author Molly Stevens leans on quick-charred scallions instead. Chipotle-spiked mayonnaise adds an extra dose of lushness and a punch of smoky heat. For the cheese, semisoft cheeses like Oaxaca or Monterey Jack add a satisfying tang, but any good melting cheese works—cheddar or Swiss are perfectly delicious stand-ins. You’ll need a heavy skillet (cast iron works great) that can hold four sandwiches; otherwise work in batches or in two skillets. Form the ground beef patties to match the size of the bread, ensuring every bite of the finished sandwich achieves a proper bread-to-cheese balance.