How to Make It
Heat a cast-iron griddle until very hot. Layer the pickle slices on the bottom buns.
Without overworking the meat, loosely form it into 4 balls and place them on the griddle. Cook the meatballs over moderately high heat for 30 seconds. Using a sturdy large spatula, flatten each ball into a 5-inch round patty. Season the patties with salt and pepper and cook for 2 minutes, until well seared. Press a handful of sliced onions onto each patty. Using the spatula, carefully flip each burger so the onions are on the bottom. Top with the cheese and cook for 2 minutes. Cover with a roasting pan and cook just until the cheese is melted, 1 minute more. Transfer the burgers with the onions to the buns and sprinkle with umami dust. Top with the buns and serve.
The Secret Seasoning To make simplified umami dust, use a spice grinder to pulse 3 tablespoons bonito flakes, 1/2 ounce crumbled dried kombu, and 1/2 ounce dried shitake mushrooms into a powder. Umami dust will soon be available at umamiburger.com.