This is raw mekabu. It's located at the root of the wakame seaweed plant. It's already exuding its sliminess. Wash well.
Blanch in salted water and it'll turn bright green.... Cool rapidly in cold water to fix the bright color.
Chop up with a kitchen knife. It will get slimier and slimier. I recommend a rhythmic pop song playing in the background as you chop it up.
Add bonito flakes, sesame seeds, chopped green onion, soy sauce and egg yolk (optional) and mix well. If this is for adults, add some wasabi.
Marinate the tuna for 30 minutes to an hour. The marinade is soy sauce, mirin and sake at a 2:1:1 ratio with a piece of kombu soaked until softened (the photo shows 6 servings' worth).
Put the mekabu seaweed on top of a bed of piping hot rice, and add the marinated tuna on top. Add shiso leaves and shredded nori seaweed to taste, and serve with wasabi soy sauce. The bowl on the right is the more standard tuna and yam rice bowl.
I like to cut corners, so I always have this over hot plain rice, but you can have this with sushi rice if you prefer -- it's delicious this way, too.
Story Behind this Recipe
When I visited a restaurant in a fishing village called Kaneda on the Miura Peninsula in Kanagawa prefecture, they had a rice bowl with grated nagaimo and tuna over rice, as well as an "Ocean Bowl" (tuna and mekabu over rice) on their menu. I was intrigued by the name and ordered it, and found out it's one of those sticky dishes that are so popular these days. It seems healthy! Since then, it's become a favorite in our family. It's delicious with natto mixed with the mekabu seaweed too. Natto and tuna go well together after all.
If you can get a hold of raw mekabu seaweed in the spring, definitely use that for this. It is so much stickier and tasty than the kind you can get at the supermarket. Off season, just use the pre-seasoned mekabu seaweed that's packaged in portions.