Drizzle the salmon, nectarines, and onions with olive oil and sprinkle with salt.
Heat an outdoor grill or grill pan to medium heat. Grill the salmon, skin side up, for 10-15 minutes, flipping halfway through, or until the fish flakes easily with a fork.
At the same time, grill the onions and nectarines for 10-15 minutes on medium heat, flipping once.
Meanwhile, make the vinaigrette by combining the basil, garlic, mustard, honey, salt and vinegar in a blender or food processor. Blend on high until the dressing starts to come together, then slowly drizzle in the olive oil. Continue to blend until the vinaigrette is smooth.
Assemble the salads by placing 2 cups of greens on each plate, then topping with the salmon, nectarines, and onions.
Drizzle with vinaigrette, and sprinkle with additional salt and freshly ground pepper to taste. Store leftover vinaigrette in the fridge, tightly covered, for up to 3 days.