In a small saucepan, combine the stock, two soys, and syrupy rice wine. Heat this mixture until it just begins to simmer. Sprinkle the bonito flakes over the seasoned stock. Remove the saucepan from the heat and let the stock steep for 3 minutes before pouring it through a cloth- or paper-lined strainer or colander. With the back of a spoon, press on the solids to extract all the liquid from the bonito flakes. Let the seasoned stock cool to room temperature, then add the rice vinegar to complete making the smoky vinaigrette. Cover the dressing and chill for at least 30 minutes before serving. You may prepare this step ahead of time. The dressing keeps for up to 1 week in the refrigerator.
Rinse the fillet under cold water and pat dry completely. Lay the fillet on your cutting board and, with a long-bladed, very sharp knife, slice the fish into broad, thin, diagonal slices. Your knife should be held at a 45-degree angle to your board and the blade should be pulled smoothly through the fish, not sawed back and forth. If you succeed in producing eight or ten gorgeous slices (and you will—with a bit of practice), you can arrange them to look like a rose. If the slices should fall apart or not measure at least 4 inches in length and
Rinse the lettuce leaves under cold water and shake them free of excess moisture (a few remaining “dew drops” are fine). Place the leaves on your serving plates.
To make roses from your slices, roll the fish as illustrated. Place two flowers on each lettuce leaf. With a blunt point, open up the center of the flowers just enough to allow a few bits of chive or scallion green to nestle in each. If you’re serving the fish in julienne shreds, toss them with the chives or scallion greens, then mound them on the leaves. In either case, moisten the fish with a few drops of the reserved smoky vinaigrette dressing just as you get ready to bring the plates to the table.