Pat the mahi-mahi fillets dry with paper towels. If fillets are thicker than 1 inch, slice into 1-inch-thick portions for even cooking. Brush both sides lightly with 1 tbs olive oil.

Make spice mix

In a small bowl combine 1 tsp paprika, 1 tsp smoked paprika, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp dried oregano, 1/2 tsp ground cumin, 1/4 tsp cayenne pepper, 3/4 tsp kosher salt, and 1/2 tsp freshly ground black pepper. Mix thoroughly.

Season fish

Generously press the blackening spice mix onto both sides of each fillet so they are well coated.

Cook fish

Heat a large cast-iron or heavy skillet over medium-high heat until smoking hot. Add 1 tbs unsalted butter and 1 tsp neutral cooking oil to the pan. When butter is melted and foaming, add the seasoned fillets. Cook without moving for 2–3 minutes until a dark crust forms, then flip and cook 2–3 minutes more until the fish flakes easily and reaches 145°F (63°C). Transfer to a plate and let rest 2 minutes, then flake into bite-sized pieces with a fork.

Ranch Slaw

In a medium bowl combine 2 cups shredded green cabbage, 1 cup shredded red cabbage, 1/2 cup shredded carrot, and 2 tbsp chopped cilantro.

Add 1/2 cup ranch dressing and 1 tbs lime juice. Toss to coat evenly. Season with 1/4 tsp salt and 1/8 tsp black pepper, taste and adjust. Refrigerate until ready to assemble; slaw can be made up to 2 hours ahead.

Guacamole

Halve and pit the 2 ripe avocados and scoop flesh into a bowl. Mash with a fork to desired texture (slightly chunky is recommended).

Fold in 1 tbs lime juice, 2 tbsp finely minced red onion, 1 tsp minced jalapeño, 1 tbsp chopped cilantro, 1/4 tsp kosher salt, and 1/8 tsp black pepper. Taste and adjust seasoning; cover with plastic wrap pressed directly to the surface to prevent browning if not using immediately.

Assembly & Tortillas

Warm the tortillas: heat a dry skillet over medium heat. Lightly brush each side of a tortilla with a few drops of neutral oil if desired and warm 20–30 seconds per side until pliable and lightly toasted. Keep tortillas wrapped in a clean towel to stay warm.

Assemble tacos: place a warm tortilla on a plate, sprinkle about 2 tbsp shredded pepper jack cheese down the center, add a generous 2–3 oz portion of flaked blackened mahi-mahi, top with 2–3 tbsp ranch slaw and 1–2 tbsp guacamole. Repeat with remaining tortillas.

Serve: garnish with lime wedges for squeezing. Serve immediately while fish and tortillas are warm.