For the crispy battered fish: Preheat the oven to 200 degrees F. Fit a wire rack in a baking sheet; set aside.
In a large pot, heat 2 inches of vegetable oil over medium-high heat to 375 degrees F.
Make the batter: In a large bowl, whisk together the flour, cornstarch, salt, cayenne, paprika, baking powder and pepper. Add the beer and whisk until smooth.
Working in 2 to 3 batches, coat the fish in the batter. Carefully transfer the battered fish to the hot oil. Fry each batch for 4 minutes, flipping halfway through. Remove the fried fish to the wire rack on the baking sheet to allow the oil to drain away. This will ensure it stays crispy. Transfer the baking sheet with the fish to the oven to keep warm while you get everything else ready. The fish can sit in the oven up to an hour staying warm.
For serving: Wrap the tortillas in foil and place in the oven to warm for 10 minutes.
Unwrap the tortillas and serve alongside the fish and all the fixings: cabbage, red onion, cilantro, lime wedges and Charred Lime Crema.
Charred Lime Crema:
Heat a grill or grill pan over medium-high heat.
Toss the jalapenos and onion rounds in the olive oil in a bowl. Grill the jalapenos and onion rounds until charred all over, turning as needed, about 5 minutes total. Place the jalapenos in a bowl and cover tightly with plastic wrap for 10 minutes. Set the onions aside.
Cut the jalapenos in half, remove the stems and seeds and gently scrape off the charred skin. Roughly chop the grilled onion rounds.
Combine the jalapenos, onion, cilantro and some salt and pepper in a food processor. Pulse in 2-second increments until finely minced.
Whisk together the crema and lime zest and juice in a medium bowl. Add the jalapeno-onion mixture to the bowl with the crema and fold everything together until fully incorporated.
Pour into a serving dish and garnish with cilantro leaves. Serve as a flavorful topping on your favorite tacos.