Steam the bao buns over boiling water for about 6-8 minutes. You can use a bamboo steamer or a colander with a lid over a pot. Make sure the buns don't come into direct contact with the water.
Pat the tuna fillet dry with paper towels, season with salt and pepper, then sear in a hot pan with a little oil for 1 minute on each side, so that the outside is browned but the inside remains pink. This is the essence of the 'tataki' style preparation.
In a small bowl, mix the mayonnaise with the wasabi paste. Add more wasabi to taste if you want it spicier. This will be the piquant sauce.
Thinly slice the red onion and finely shred the cabbage. Toss them together with soy sauce and sesame oil. Let it stand for 5 minutes to allow the flavors to meld.
Carefully open the steamed bao buns. First, spread a little wasabi mayonnaise inside, then add a few slices of seared tuna, and finally spoon over the cabbage and red onion mixture.
Serve immediately while the bun is still warm and the tuna is fresh. Before serving, drizzle with a little more soy sauce or sesame oil if you want to intensify the flavors.
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