Make the slaw first. Toss the shredded green and red cabbage with cilantro, lime juice, vinegar, and a pinch of salt. Mix well and set aside to rest while you prepare everything else.

Mix the sauce. Stir together the sour cream, mayonnaise, lime juice, hot sauce, and garlic powder. Taste and add salt if needed. Refrigerate until ready to use.

Prep the fish. Pat the cod fillets completely dry with paper towels. Cut into strips roughly 3 inches long and 1 inch wide.

Make the batter. Whisk together the flour, cornstarch, garlic powder, smoked paprika, cumin, salt, and pepper. Slowly stir in the cold sparkling water or beer until you get a batter that coats the back of a spoon. A few lumps are fine — don't overmix.

Heat the oil. Pour about 1/2 inch of vegetable oil into a heavy pan over medium-high heat. Test by dropping a small bit of batter in — it should sizzle immediately.

Fry the fish. Dip each strip into the batter, let excess drip off, and lay gently into the hot oil. Fry in batches, about 2 to 3 minutes per side, until golden and crispy. Drain on a paper towel-lined plate.

Warm the tortillas. Heat in a dry pan over medium heat for 30 seconds per side, or wrap in a damp paper towel and microwave for 45 seconds.

Build your tacos. Layer slaw, fried fish, creamy sauce, and avocado slices onto each tortilla. Finish with a squeeze of fresh lime and sliced jalapeño if desired. Serve immediately.