Special equipment: a deep-frying thermometer
For the batter: Mix the flour, mustard, oregano, 1 teaspoon salt and 1/2 teaspoon pepper in a medium bowl. Gradually add the beer while whisking; the beer will make your batter crispier and more airy. Whisk until combined, then set the batter aside.
For the cod: In a large deep skillet over medium heat, add enough oil to cover 1 inch of the skillet and heat until the oil reaches 350 degrees F.
On a large plate, combine the flour and 1 teaspoon salt. Season the fish strips with salt and pepper. Coat the fish in the flour mixture, then dip them into the beer batter. Fry in batches in the hot oil until golden brown and cooked through, about 5 minutes. Transfer to a rack for the excess oil to drain.
For the Brussels sprouts slaw: In a bowl, add the olive oil, vinegar, chili oil, oregano and sugar and whisk to combine. Add the Brussels sprouts and toss to coat. Season with salt and pepper.
For the tacos: Heat the tortillas directly over a gas burner until charred in spots but still pliable. Make the tacos with the warm tortillas and fish, then top with the crema, Brussels sprouts slaw, salsa, red onions and jalapeno.