Tuna Tartare with Ginger and Toasted Sesame

This version of tuna tartare uses ingredients from the sushi counter, pickled ginger, rice vinegar, which is usually used in making sushi rice, and a little soy sauce added to the finely chopped fish. Place it on small plates as an appetizer or in a bowl along with taro or other crispy chips for a cocktail party. (It's best served right away; while the tuna is cooking, it starts to lose color, although the taste remains quite pleasant.) Look for pickled ginger next to ready-made sushi at your local grocery store or in the Asian food section.
Combine the first 5 ingredients in a medium bowl. Lay out the tuna. Spoon onto serving plates and sprinkle with sesame seeds.