Position oven rack 4-6 inches away from heat source. Preheat broiler to high. Line a baking sheet with foil and spray with cooking spray.
Mix ½ tbsp mustard, ½ tsp parsley, ¼ tsp salt, and ⅛ tsp pepper in a bowl. Place salmon skin-side down on baking sheet and coat with sauce. Broil until opaque, about 6-7 minutes or until reaches an internal temperature of 145F.
Meanwhile, to prepare vinaigrette, mix vinegar, garlic powder, maple syrup, ¾ tsp mustard, ½ tsp parsley, ¼ tsp salt, ⅛ tsp pepper and onions in a bowl. Slowly add olive oil whisking vigorously to ensure vinaigrette is well-combined.
To serve, toss cherry tomatoes, spinach, and lettuce with vinaigrette and top with avocado and salmon.
