Place cucumbers in a colander and toss with salt; let stand until several tablespoons of liquid has drained, 30 to 45 minutes. Pat dry with paper towels and transfer to a medium bowl. Add onion, sour cream, vinegar, dill and a few grinds of pepper. Toss to coat. (Can be refrigerated several hours ahead.)
Meanwhile, heat gas grill, with all burners on high, until fully preheated, 10 to 15 minutes. Use a wire brush to clean grill rack, then brush lightly with oil. Close lid and let return to temperature. Brush salmon with oil and season with salt and pepper; grill until just opaque, about 3 minutes per side.
Set each warm or room-temperature salmon fillet on a plate; spoon cucumber salad over part of and alongside the fish.
