Heat 2 tablespoons of oil in a large skillet over medium-high heat. Add the chile pepper and the fish and sprinkle with salt and pepper. Saute until the fish is just cooked through, about 2 minutes. Remove from the pan and turn the heat to high.

Add 1 tablespoon of oil to the skillet with the cabbage, 1 tablespoon of water, and salt and pepper. Cook, stirring until the cabbage begins to wilt, about 5 minutes. Turn off the heat and add the cilantro, lime juice, chili powder (if using), and fish. Taste and adjust the seasonings.

Warm the tortillas one at a time in a dray skillet over medium heat, just a few seconds each side. Scoop the fish cabbage mixture into each tortilla and top with tomato and avocado.