In a blender, combine the avocado half with the roasted poblano and jalapeño chiles, the 1/4 cup olive oil, 1/4 cup of the chopped onion, 1/4 cup of the cilantro leaves, the lime juice, 1/4 teaspoon of the kosher salt, and the water. Puree until smooth, and set aside.
Preheat a grill to medium, or preheat a cast-iron grill pan over medium heat.
Season the amberjack with the remaining 1 teaspoon kosher salt and the white pepper. Rub the remaining 2 tablespoons olive oil over both sides of the fish, and place the fish on the grill. Grill until the fish is just cooked through and flakes easily, about 4 minutes on each side. Remove the fish from the grill, and use a fork to break it into flakes or small chunks.
To assemble the tacos, divide the fish evenly among the tortillas. Place 1 tablespoon of the remaining chopped onion and 1 tablespoon of the remaining cilantro leaves over each portion of fish. Place a half tomato slice over each, and drizzle with some of the avocado salsa. Squeeze some lime juice over the tacos, fold the tortilla sides together, and serve immediately.
Makes 8 tacos, 4 servings