Preheat oven to 400 degrees F. Place onion in medium bowl. In small saucepan, stir together vinegar, sugar, 1/2 teaspoon salt, and 1/4 cup water. Heat to boiling over medium high heat. Stir until sugar is fully dissolved, then pour vinegar mixture over onions and let stand at room temperature, stirring occasionally, as you continue recipe. 

Sprinkle salmon with chili powder, cumin, 1/2 teaspoon black pepper and remaining 1 teaspoon salt.

Heat oil in large oven-safe nonstick skillet over medium heat. Transfer salmon, flesh side down, to skillet. Cook 3 to 4 minutes, undisturbed, until salmon flesh is deep golden brown.

Flip salmon so it is skin side down, then transfer pan to oven and continue cooking salmon until it reaches an internal temperature of 145 degrees F. How long this takes will depend on the thickness of your salmon piece(s); the salmon could take anywhere from 5 to 15 minutes in the oven to reach temperature. 

Meanwhile, add cabbage, cilantro, 1/2 cup Fish Taco Sauce, and remaining 1/2 teaspoon ground black pepper to large bowl and toss until well combined. 

Drain onions and discard pickling liquid. 

When fish is finished cooking, slip spatula between salmon skin and flesh to remove skin. Discard skin and break salmon into large chunks and arrange over tortillas. Top with cabbage mixture, pickled onions and avocado, and serve with remaining Fish Taco Sauce for drizzling.