Season salmon with 2 teaspoons of the Dijon, 1/2 teaspoon dried parsley, 1/4 teaspoon salt and black pepper.
Adjust the oven on the second rack. Broil salmon 6 to 7 minutes, until cooked through.
In a large bowl, combine the red onion with olive oil, 1 1/2 tablespoons apple cider vinegar, 1 teaspoon remaining Dijon, garlic powder, 1/4 teaspoon parsley, 1/4 teaspoon salt and pepper to taste; let it sit about 5 minutes, so the flavor of the onion mellows.
Add the tomatoes, avocado and toss. When ready to serve, toss in chopped lettuce and cabbage, finish with the remaining 1/2 tablespoon of vinegar, taste for salt and pepper and adjust as needed.
Divide the salad in 4 bowls and top each with salmon.