Make the Creamy Cilantro-Jalapeno Sauce by putting in a blender jalapenos, garlic cloves, onion, lemon juice, water, mayo, cilantro and salt and pepper. Blend until smooth. Refrigerate any leftovers for up to 4 days.
Make the cabbage garnish by mixing in a bowl cabbage, tomatoes, radishes, vinegar and lemon juice (or pickled jalapeno juice) and salt and pepper. Have all your ingredients chopped/sliced/shredded in advance so by the time you are ready to serve the tacos you just dump garnish ingredients into a bowl and mix.
Salt and pepper the fillets. If necessary, cut them in half crosswise to fit the size of a 6" tortilla. In a big bowl, using a mixer, whisk on low the flour, baking powder, oregano, mustard, dark beer, egg and salt and pepper or granulated broth. Whisk until there are no lumps left. The batter should not be too runny or too dense; it should be similar to the consistence of pancake batter. Set aside and let it rest while oil is heating.
Pour oil in a deep frying pan or skillet and heat over medium-high heat. You can also use your deep fryer. In that case, pour enough oil and heat until oil shimmers. It is important to achieve an even brown batter and keep an eye on the fillets so that they don't burn, just brown.
Dip each piece of fish into the batter and cover thoroughly. Once the oil is very hot fry each strip until evenly browned. Set a paper kitchen towel on a large plate to absorb grease. Keep warm.
To assemble the tacos: Warm up the tortillas, place a strip of fried fish on the tortilla and top with garnish and sauce.