Cut each fish fillet down the seam to separate the thin half from the thick half. Cut each of the pieces in half crosswise (making a total of 8 pieces). Sprinkle the fish pieces evenly all over with the salt, transfer to a resealable bag, and add the lime juice and oil. Chill for 30 minutes.
While the fish is marinating, finely crush (it should not be a powder) the Tortilla Strips or the store-bought tortilla chips (and toss the store-bought crushed chips with the chili powder).
Combine the red onion, vinegar, orange juice, sugar, salt, cumin, and oregano in a small saucepan, bring to a boil, and simmer for 5 minutes. Let cool. Drain the mixture and chop the onions. Combine them with the jalapeños, if using, in a small bowl.
Preheat the oven to 375°F. Arrange the fish in a single layer on a rimmed baking sheet, keeping the thin pieces on one side and the thick pieces on the other. Pack the tortilla crumbs on the top of the fillets, drizzle the crumbs with the melted butter, and bake the fish on the middle rack of the oven until it is just cooked through, about 5 minutes for the thinner pieces and 8 minutes for the thicker pieces.
Transfer a thick and thin fillet piece to each of 4 plates. Serve each portion topped with some of the crema, pickled onions, and pickled jalapeños.
Note: A kind of Mexican sour cream, crema is increasingly available at American supermarkets. If you can’t find it, sour cream or crème fraîche, diluted with a bit of milk, cream, or water, will do.
From Sara Moulton's Home Cooking 101 by Sara Moulton. Copyright © 2016 by Sara Moulton. Reprinted by permission of Oxmoor House, an imprint of Time, Inc.