Separate stems and leaves from cilantro (you should have about ½ cup of each); coarsely chop leaves. Purée cilantro stems, sour cream, lime zest, and lime juice in a blender.

Transfer cilantro mixture to a large bowl and whisk in mayonnaise; season with salt and pepper. Toss in cabbage, jicama, scallions, and chopped cilantro leaves.

Preparation

Cook chiles and garlic in a large saucepan of boiling salted water until chiles turn bright red, about 2 minutes; drain. Blend chiles, garlic, and vinegar in a blender until almost smooth; season with salt. Let cool.

DO AHEAD: Hot sauce can be made 1 week ahead. Cover and chill.

Remove any pin bones from fish fillets (using tweezers makes this easy). Cut each fillet in half lengthwise. Cut each half on a diagonal into 1” strips. (Work with the natural shape of the fish as you cut; this will help the pieces stay together instead of falling apart when frying.)

Whisk all-purpose flour, rice flour, and salt in a medium bowl. Gradually whisk in club soda until no lumps remain; adjust with more club soda or rice flour as needed to make it the consistency of thin pancake batter—it should be pourable, but thick enough to coat the fish.

Fit a large pot with a deep-fry thermometer and pour in oil to measure 2”. Heat over medium-high heat until thermometer registers 350°.

Working in batches of 5–7 pieces at a time, coat fish in batter, letting excess drip off, then carefully place in oil (to avoid splattering, lower fish into oil pointing away from you). Don’t overcrowd the pot: The oil temperature will drop dramatically and fish may stick together.

Fry fish, turning occasionally with a fish spatula or slotted spoon and maintaining oil temperature at 350°, until crust is puffed, crisp, and golden brown, about 5 minutes. Transfer to a wire rack set inside a rimmed baking sheet; season immediately with salt.

While fish is frying, use tongs to heat tortillas one at a time directly over a gas burner, moving them often, until lightly charred and puffed in spots, about 1 minute per side. Transfer to a plate; cover with a clean kitchen towel to keep warm. (If you don’t have a gas stove, wrap up a stack of tortillas in a sheet of foil and heat in a 350° oven until warmed through.)

Top tortillas (we like two per taco) with fish, slaw, hot sauce, avocado, cilantro, and jalapeños. Serve with lime wedges.