Heat broiler. Place onion and scallions in a bowl of ice water; soak 20 minutes.
Place salmon fillets on a baking sheet. Rub oil over all fillets; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper. Broil, skin side down, until salmon is opaque throughout, 8 minutes.
Drain onion and scallions, squeezing in paper towels to remove as much liquid as possible; transfer to a bowl. Add sugar, vinegar, and a pinch each of salt and pepper. Serve relish with salmon.