For the mayonnaise, beat the egg yolk in a bowl with the mustard powder and vinegar. Gradually add the oil at a constant pace until the mixture emulsifies. Once the mayonnaise is thick, add the honey and salt.
For the salad, drain the wakame that has been soaking in cold water for 20 minutes, dry it and cut into 2cm pieces. In a bowl, mix the daikon (chopped julienne), wakame seaweed and toasted sesame seeds.
Place the daikon and wakame on top of the tuna. Dress everything with the mayonnaise and serve.