Baja-Style Tempura Fish Tacos

In a large bowl, combine the cold water, lime juice, garlic, chiles, oregano (if using) and salt. Add the fish and set aside at room temperature for 20 minutes.

In a separate bowl, whisk together the ice water and mustard. Gently stir in the flour, being careful not to overmix the batter; a few small lumps are okay. Cover and refrigerate for 30 minutes.

Drain the fish and pat the pieces dry with a paper towel. Have a plate lined with paper towels or a cut-open brown paper bag at the ready.
In a large, heavy-bottomed pot, heat at least 2 to 3 inches of oil over medium heat until it reaches 360°F (182°C) on a deep-fry or candy or instant-read thermometer.
Remove the tempura batter from the refrigerator and stir to recombine.
Dredge a few pieces of fish at a time in the batter, turning to coat them evenly. Gently lower a couple pieces of fish into the oil, adding a couple more pieces every 30 seconds. Fry no more than 4 pieces at a time. Fry the fish, turning as needed, until the pieces are crisp, light golden brown, and floating on the surface of the oil, about 2 1/2 minutes. Monitor the temperature of the oil throughout frying, letting the oil return to proper temperature between batches; to ensure crispness, it must remain a constant 360°F to 380°F (182°C to 193°C).
Using a fine-mesh skimmer, transfer the fried fish to the paper towel-lined plate or brown paper bag to drain. Repeat with the remaining fish and batter, being sure to remove any flecks of floating fried batter in between batches.

Place the tortillas side by side, open face and overlapping, on a platter. (If using extra tortillas, simply double them up.) Divvy the slaw and fish equally among the tortillas. Top with salsa and desired garnishes or allow guests to do so themselves. Grab, fold, and eat right away.