Cut salmon into 1” cubes. Place in a bowl; coat with oil and dry glaze. Thinly slice lemon into rounds. Cut onion into chunky pieces. Thread salmon, lemon, and onion onto two or four skewers.
Place skewers on grill; turn occasionally, until grill marks form and salmon is cooked through, about 5–8 min.
Place greens in a bowl and toss with dressing. Divide onto plates and top each with one skewer, or remove salmon from skewers and place over greens. Top with avocado, if using.