Step 1

Whisk 1 cup sour cream or plain whole-milk Greek yogurt, 2 Tbsp. sriracha, 1 Tbsp. fresh lime juice, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ¼ tsp. sugar, and 2 Tbsp. water in a small bowl to combine. Cover and chill sauce until ready to use.Do ahead: Sauce can be made 1 day ahead. Keep chilled.

Step 2

Toss ¼ small head of cabbage, thinly sliced, 2 Tbsp. fresh lime juice, 1 Tbsp. extra-virgin olive oil, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl to combine. Set slaw aside.

Step 3

Mix 1 Tbsp. ground cumin, 1 Tbsp. paprika, ½ tsp. cayenne pepper, and remaining 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt in a small bowl to combine.

Step 4

Place 1½ lb. skinless, boneless cod or halibut fillets on a small baking sheet or plate and sprinkle spice mix over to coat. Drizzle remaining 2 Tbsp. extra-virgin olive oil over fish and rub with your hands so a light paste forms on fish with no dry spots.

Step 5

Heat a dry nonstick skillet over medium-high. Cook fish, gently turning over with a fish spatula halfway through, until golden and flesh is opaque and flakes easily with a fork, about 12 minutes total. Remove from heat.

Step 6

To serve, top warm small corn tortillas with pieces of fish and reserved slaw. Top with thinly sliced red onion, thinly sliced jalapeño, and cilantro leaves with tender stems. Serve with lime wedges for squeezing over.