The lamb burgers sit on a bed of sweet and spicy caramelized red onions and jalapeños and the bun is slathered with a bright, zingy herb sauce that can be made with either Greek yogurt or mayonnaise (or my favorite, a combination of both).
Stir together cilantro, scallion, mint, lemon juice, lemon zest, and yogurt (or mayonnaise) until combined.
Combine the lamb, cumin, Sichuan peppercorns, chile flakes, sesame oil, sugar, mustard and salt in a large bowl. Mix gently until spices are evenly distributed but try not too compress the meat too much. Form into four equal-size balls, cover with cling film and hold them in the refrigerator while you cook the onions.
Preheat a large cast-iron or other heavy-bottomed skillet on medium-high. Add 1 tablespoon oil to the pan and sauté the onions and jalapeños until brown and fully cooked, approximately 10-15 minutes. Remove from pan and set aside.
In the same pan, turn the heat up to high and add the second tablespoon oil. Place the meatballs in the pan with several inches of space between them (use two pans or cook in batches, as necessary). Use a wide, stiff spatula (or the bottom of a pot) to smash the balls into flat patties, about ½ inch thick. Cook until the bottom of each patty has formed a deep brown crust, approximately 3 to 4 minutes. Flip each patty, and cook through, approximately 3 minutes for medium.
Slather herb sauce onto both sides of each bun, add some caramelized onions on the bottom buns and place a burger on each. Top with extra cilantro, jalapeño, lettuce and tomato, if desired.