Place the ground beef in a shallow bowl. Add the salt, a few grinds of black pepper, the mayonnaise and the panko crumbs. The mayonnaise and panko combination helps to hold the juices in the burger during cooking, but doesn’t change the flavor or texture of the beef. The coarse texture of the panko crumbs is important because they don’t add density to the burger like dry breadcrumbs would.
Use a wooden spatula to combine the meat ingredients as best you can before doing a final, gentle mix with your hands. Gently form the patties into quarter-pound portions and make a slight depression in the center of each with your thumb. This helps the burgers maintain an even thickness rather than developing a thicker “dome” in the center. Transfer to a plate and chill for at least 15 minutes before grilling.
While the burgers chill, combine the ketchup, tomato paste, vinegar, maple syrup, grated onion and a few grinds of black pepper in a small saucepan or microwave-safe bowl. Warm over low heat or at 50% power. Stir until smooth, cover and set aside.
Preheat a gas or charcoal grill to a medium-high temperature (about 400°F). Oil the grates, add the burgers and grill until you see juices start to accumulate on the top of the meat, approximately 4 minutes. Flip them over and top with cheese after about 1 minute. Cook for another 1 to 2 minutes, to desired doneness. While the burgers rest, lightly toast the buns on the grill.
To serve, place a burger on each bun, top with onions and Maple Ketchup.