Step 1: In a large bowl, combine 4 spring onions finely chopped, ¼ cup minced ginger, 1 beaten egg and 2 ½ cups yellow fin tuna. Season with salt and pepper to taste. Form the mixture into 6 patties (they will be quite loose), set on a plate and cover with plastic wrap. Put the patties in the fridge for at least 15 minutes to firm up a little.
Step 2: Meanwhile, slice 1 cucumber into wide ribbons using a peeler, set aside.
Step 3: Make the wasabi ginger mayonnaise by stirring together: ½ cup mayonnaise, 2 tablespoons wasabi and 6 tablespoons finely chopped pickled ginger. Add a little water if needed to thin the mayo. Cover and set aside.
Step 4: Set a large, nonstick pan over medium-high heat and add 1 tablespoon oil. When the oil is hot, add the burgers to the pan in batches. Cook until golden brown on the bottom, about 3 minutes.
Step 5: Toast 6 buns and serve the burgers on top, with a generous dollop of the wasabi mayonnaise, a handful of broccoli sprouts and a few ribbons of cucumber.