Put tuna in a chilled bowl. Add chopped scallion, pickled ginger, soy sauce and pepper. Mix all of the ingredients lightly. Depending on the size burger, you want to divide the mixture into 2 or 4 patties and form your burgers. Heat your grill or skillet over medium flame. Place brioche slices on grill and toast. Remove slices and place patties on grillpan sear to your desired doneness. Rare to well.