Separate the egg (reserve yolk for another use)
Rinse the fish fillet, pat dry and cut into small portions
Chop finely in a food processor (or with an immersion blender)
Mix fish in a bowl with 1 tablespoon breadcrumbs and the mustard, then season with salt and pepper
Form fish mixture into 4 small meatballs with moistened hands, place on a baking sheet and freeze until firm, about 5 minutes
Drain the pickled ginger and finely chop
Stir with the mayonnaise in a bowl
Cook soba noodles in a pot of boiling salted water according to package instructions
Meanwhile, squeeze lime
Mix 2 teaspoon lime juice with wasabi paste, sesame oil and soy sauce in a bowl, then season with salt and pepper
Drain the soba noodles in a sieve and rinse briefly under cold water
Toss noodles in the bowl of wasabi sauce
Mix breadcrumbs with the remaining sesame seeds on a plate
Roll meatballs in mixture and press to adhere
Heat oil in a pan over medium heat
Cook the meatballs until golden brown, 3-4 minutes on each side
Serve with noodles and ginger mayonnaise