In a small bowl, mix together the soy sauce, rice vinegar, mirin, and sesame oil.
Drain the sardines and gently place them in the marinade. Let them sit while you prepare the rest of the ingredients to absorb the flavours.
Bring a small pot of water to a boil. Blanch the edamame beans for 3 minutes, then drain and rinse under cold water to cool. Transfer to a bowl and add the sliced spring onions.
Toast the flatbreads, sourdough rolls or bagels until golden and warm. Place them on two serving plates.
Add the rocket leaves to the bowl with the edamame and spring onions. Pour over the remaining marinade from the sardines and toss gently to combine.
Arrange the marinated sardines on top of the toasted bread. Spoon the edamame and rocket salad over or alongside.
Sprinkle with toasted sesame seeds and serve immediately.