Mix tomato, tuna, sesame oil, dashi(menstuyu, soy sauce, sugar), half of the shiso leaves in a bowl. Season with salt and pepper.
Bring a large pot of water to a boil, add the somen noodles and cook according to the directions on the package. Drain the noodle and place immediately under cold running water to halt cooking and wash in ice water. Drain well and put in a serving bowl.
Pour the mixture from step ① over the somen noodles. Arrange the eggplant, asparagus, egg, and shiso leaves on it. Sprinkle pepper on top.