Mellow the Onions
Submerge the thinly sliced red onions into a bowl filled with ice water. Let them soak for 10–15 minutes while preparing the remaining elements. Drain thoroughly and pat completely dry with paper towels before using.
Raw red onion contains harsh sulfurous compounds that dominate the delicate flavor of fresh fish. Ice-soaking leaches out the harsh burn while preserving an exceptionally crisp snap.
The onions will curl slightly and take on a vibrant, translucent pink sheen.