Cold Poached Tilefish with Mustard-Miso Sauce and Fiddleheads

Here is another poached fish dish, perfect for a warm-weather menu. The mustard adds zing to an otherwise mellow sauce. The Japanese often serve this poached and sauced fish in the spring, when edible fronds and bracken such as warabi and kogomi are in season. Lately fiddlehead fern fronds have appeared in American markets, and they would be lovely served with the fish—as would barely blanched fresh green asparagus, if you can’t find the more exotic fiddleheads.