How to Make It
In a large glass or stainless-steel baking dish, spread 1/4 cup of the pickled ginger in an even layer. Sprinkle the ginger with 2 tablespoons of the pickling juice from the jar. Lay the onion slices on top of the ginger in a single layer. Cover the onion slices with the remaining pickled ginger and pickling juice from the jar. Cover with plastic wrap and refrigerate for at least 3 hours or overnight.
Meanwhile, pat the ground meat into 6 burgers and refrigerate until chilled.
In a small bowl, stir the wasabi powder into the boiling water. Whisk in the mayonnaise and refrigerate. In another small bowl, whisk the miso with the honey, soy sauce, rice vinegar and sesame oil.
Light a grill. Remove the onion slices from the pickling mixture. Generously season the sirloin burgers with salt and pepper. Grill the burgers over a hot fire for 3 to 4 minutes per side for medium rare. Spread the wasabi mayonnaise on both sides of the buns. Lay the pickled onion slices on the bottom buns and set the burgers on top. Spoon the miso sauce over the burgers, close the sandwiches and serve.
The ginger-pickled onions, wasabi mayonnaise and miso sauce can be refrigerated separately in airtight containers overnight.