For the ginger ponzu, simply whisk together the soy sauce, mirin, lemon juice and ginger. Season with a little salt. Set aside.
Cook the soba noodles in a large pot of boiling water according to the directions on the packet. Drain and rinse under cold water, then drain again. Transfer to a bowl and set aside.
Toss the prawns with the sesame oil in a small bowl. Season with a little salt. Set aside.
Divide the noodles among four bowls, topping each bowl with some prawns, avocado and cucumber slices. Pour over some of the ginger ponzu, then finish with a scattering of spring onions and sesame seeds. Alternatively, combine everything in a large bowl, toss to mix and serve.
