Place the kombu seaweed, cucumber and fine sea salt into a bowl with cold water and leave to soak for 10 minutes. After this time, drain and dry them.
For the egg yolk sauce, mix the mirin, yolks, soy sauce and vinegar in a stainless-steel bowl. Place this bowl over a pan of boiling water, making sure that the bowl does not touch the water. Heat the mixture over a low heat stirring continually until it starts to thicken slightly. Finally, strain the sauce through a fine sieve and set aside to cool.
Carefully separate the tuna into chunks.
Place the sliced cucumber and tuna chunks alternately on a plate. Cover them with the sauce, then serve.