Soak onion in 1 cup water with 1/2 teaspoon salt 15 minutes, then drain. Stir together onion, tomato, vinegar, sugar, and remaining 1/4 teaspoon salt and let stand 20 minutes.
Drain relish in a sieve, discarding liquid, then stir in dill.
Pat scallops dry and season with salt and pepper. Grill scallops in 2 batches on lightly oiled grill rack, turning once, until just cooked through, 4 to 5 minutes.
Serve scallops with relish.
Scallops can be grilled in a hot lightly oiled well-seasoned ridged grill pan over high heat.To serve as an hors d'oeuvre, top each scallop with 1 teaspoon relish.