Soak 1/2 cup finely chopped red onion in 1 cup water with 1/2 teaspoon salt 15 minutes, then drain. Stir together onion, 1 lg seeded & chopped tomato, 4 tsp red wine vinegar, 1 tsp sugar, & 1/4 teaspoon salt and let stand 20 minutes. Drain relish in a sieve, discarding liquid, then stir in 2 tbsp chopped fresh dill. Cook scallops while relish is standing: Pat scallops dry and season with salt and pepper. Grill scallops on lightly oiled grill rack, turning once, until just cooked through, 4 to 5 minutes. Serve scallops with relish.