These fish tacos are a traditional Mexican dish that's sure to be a crowd-pleaser. A quick lime juice marinade for the cod provides a hint of citrus in each bite, and the beer batter is wonderfully crisp and light. Simple, rustic, delicious: it's everything you want from Mexican street food.
Marinate: Slice fish into 1-inch-thick pieces (about 2 ounces each). Place in a wide, shallow bowl and season with salt on all sides. Pour lime juice over the pieces and let rest while preparing the batter, 5–10 minutes.
Make batter: In a medium bowl, whisk together flour and beer until smooth. Whisk in baking powder, then add sambal, mustard, oregano, and salt, and whisk to combine. Set aside.
Dredge, batter, and deep-fry: Place flour on a plate. Add oil to a pot and set over medium heat. Secure a thermometer to the pot’s side and heat oil to 375 F. Remove fish from marinade, one piece at a time, and dredge in flour on both sides. Then dip in the batter, coat well, and carefully place into the hot oil. Repeat with one or two more pieces, working in batches to avoid crowding the pot. Fry until the fish is evenly cooked and golden brown on all sides, about 8 minutes. Transfer fish to a paper towel-lined plate.
Assembly: Warm tortillas on a comal or skillet over low heat. While tortillas are heating, mix together mayonnaise and crème fraîche in a small bowl. Place a warm tortilla on a plate, and spread a dollop of mayo-crème sauce down the middle. Place a piece of fish on top of the sauce, followed by another spoonful of sauce. Add cabbage, an avocado slice or two, then finish with a spoonful of pickled red onions. Repeat assembly with remaining ingredients and serve immediately.