In a small bowl, combine paprika, smoked paprika, garlic powder, onion powder, dried oregano, dried thyme, cayenne, salt, and black pepper to make the blackening spice blend. Sprinkle the blend evenly over both sides of the fillets, pressing gently to adhere.

Heat a large skillet (cast-iron preferred) over medium-high heat until very hot, about 3–4 minutes. Add 1 tablespoon unsalted butter and swirl to coat.

Add fillets to the pan and sear without moving 2–3 minutes until a dark crust forms. Flip and cook 2–3 more minutes until fish is opaque and flakes easily (internal temperature 140–145°F / 60–63°C). Remove to a plate and tent loosely with foil to rest for 3 minutes.

Mango Slaw

While fish cooks, prepare the slaw. Peel and dice or thinly slice the mango into small matchsticks to yield about 1 cup.

In a medium bowl, combine mango, shredded red cabbage, julienned carrot, thinly sliced red onion, and chopped cilantro.

In a small jar or bowl, whisk together lime juice, honey, 1 teaspoon olive oil, 1/4 teaspoon salt, and 1/8 teaspoon black pepper. Pour dressing over slaw and toss gently to combine. Taste and adjust seasoning. Refrigerate briefly if making ahead; serve chilled or at room temperature.

Garlic Herb Aioli

In a small bowl, whisk together 1/2 cup mayonnaise, minced garlic, lemon juice, chopped parsley, chopped chives, 1/2 teaspoon olive oil, 1/8 teaspoon salt, and 1/8 teaspoon black pepper until smooth.

Cover and refrigerate the aioli at least 10 minutes to let flavors meld. Stir again before assembling.

Assembly

Split buns or rolls and evenly spread softened butter on cut sides. Toast the buns in a hot skillet or under a broiler until golden, about 1–2 minutes, watching closely.

Spread a generous tablespoon of garlic herb aioli on both the top and bottom bun halves (adjust to taste).

Place a lettuce leaf on the bottom bun (if using), then a portion of blackened mahi mahi. Top the fish with about 1/4–1/3 cup mango slaw per sandwich.

Close with the top bun, serve with lime wedges alongside to squeeze over fish. Serve immediately.