There's a secret to 12 Baltimore's $10 burger. It's not on the menu, and the servers don't brag about the mystery ingredient, but chef Marshall Roth is happy to reveal what it is: duck fat. "Most hamburgers are an 80-20 blend of lean beef and fat," he notes. "I only use 10 percent of beef fat and add a little duck fat to the ground beef, which makes the meat really juicy and flavorful." To get the full effect of Marshall's formula, you have to order it medium-rare, because higher temperatures cook out the fat and the burger tastes dry. Roth also believes in big burgers: 12 Baltimore's is a half-pound of beef before it goes on the grill. Patrons can have their meat covered in cheddar, Swiss or provolone cheese, and the buttery brioche bun is spread with Roth's sassy a•oli, a homemade mayonnaise made with a touch of fiery Thai sriracha sauce. "It's a labor-intensive burger," he admits, "but I really believe that if you're going to make something taste really great, why not take the extra step?"