Prepare dressing by whisking all dressing ingredients together in a blender, food processor or by hand. Set aside.
Build a medium-hot grill fire.
Drizzle salmon fillets with lemon juice and sprinkle with black pepper.
Grill salmon, skin sides up, on a grill rack coated with cooking spray. Grill for about 5 minutes on each side or until fish flakes easily when tested with a fork. Remove and discard the skin from fillets (or the skin makes a great healthy cat or dog treat).
In a large salad bowl, toss together spinach, orange sections, avocado, cucumber, and red onion with the dressing until well coated.
Divide salad among 4 large plates and top each salad with grilled salmon. Serve immediately.