Sour cream, or crumbled queso fresco or other grated cheese (optional)
Shredded cabbage, chopped tomato and cucumber, and cilantro sprigs (optional)
Lime wedges
Instructions
Put the onion and jalapeños in a large skillet with a tight-fitting lid or in a microwave-safe covered baking dish. Add 1 tablespoon water and the fish; sprinkle with salt and pepper. Cover and put over medium heat or in the microwave. Cook for about 6 minutes or microwave on high power until the fish is flaky; start checking after 3 minutes.
While the fish cooks, heat the tortillas. You can toast them in a skillet over medium heat, one at a time, turning them once until hot, a minute or so, wrap them in two equal batches in foil and warm them in a 375°F oven for 5 to 10 minutes, or enclose them in damp kitchen towels and microwave for a minute or 2.
To serve, fill the warm tortillas with a small portion of fish along with a bit of the onion and jalapeño. Top with salsa and, if you like, hot sauce, sour cream or cheese, and/or vegetables. Squeeze lime juice over all.