- Make the fennel slaw: In a mixing bowl, combine fennel, onion, lime juice, cilantro, and a pinch of salt. Toss to combine, then set aside.
- Make the chipotle crema: In a blender, combine sour cream, mayonnaise, salt, lime juice, and 3−5 chipotle peppers (depending on the level of spice you want). Process until smooth, then set aside.
- Dredge and fry the oysters: In a cast-iron skillet fitted with a deep-fry thermometer, heat oil to 350 degrees. In a mixing bowl, combine flour, masa harina, salt, and chipotle. Pour buttermilk into another bowl. Dip oysters in buttermilk and let excess drip off, then thoroughly coat in flour mixture and set aside on a plate. Once all oysters are dredged, add to oil in batches of 8 at a time. Fry until golden brown, then transfer to a paper towel-lined plate.
- Assemble tacos: While oysters are frying, heat tortillas in an ungreased pan until hot. Wrap in a dishtowel to keep warm and steam. When oysters are ready, spread about 1 tablespoon of chipotle crema on a warm tortilla and top with 3 fried oysters, about 2 tablespoons of fennel slaw, and 2 slices of radish. Repeat with remaining ingredients.