Pat fish fillets dry with paper towels. In a small bowl combine paprika, smoked paprika, garlic powder, onion powder, dried thyme, dried oregano, cayenne, black pepper, and salt. Rub each fillet lightly with olive oil, then evenly coat both sides with the blackening spice mix, pressing the rub into the flesh.

Cook fish

Heat a large heavy skillet (preferably cast iron) over medium-high heat until very hot. Add the butter and let it foam. Place fillets in the pan and sear without moving 3-4 minutes per side, depending on thickness, until a dark crust forms and internal temperature reaches 135-145°F (cook to desired doneness). Transfer cooked fillets to a plate and tent with foil to rest for 3 minutes.

Crunchy Cabbage & Greens

Meanwhile, finely shred the green and red cabbage and place in a medium bowl. Add the baby arugula. Whisk together lemon juice, mayonnaise (just a teaspoon for a light binder), salt and black pepper; toss with the cabbage and arugula until evenly coated. Chill briefly while other components are prepared so cabbage stays crisp.

Firestick Onions

Cut the yellow onion into thin matchstick pieces (about 3–4 inches long). Pat dry with paper towels to remove excess moisture. In a shallow bowl whisk together flour, cornstarch, salt, black pepper, and smoked paprika. Toss onion sticks in the flour mixture until well coated, shaking off excess.

Pour vegetable oil into a deep skillet or pot to a depth of about 1–1.5 inches and heat to 350°F (175°C). Fry the coated onion sticks in small batches for 2–3 minutes until golden brown and crisp. Use a slotted spoon to transfer to a paper towel–lined tray and season with a light sprinkle of salt while hot.

Tartar Sauce

In a small bowl combine mayonnaise, Dijon mustard, chopped capers, chopped dill pickle, lemon juice, chopped dill or parsley, and optional hot sauce. Season with salt and black pepper, taste and adjust acidity or salt as needed. Refrigerate until ready to assemble.

Sandwich Assembly

Toast the brioche buns: spread the 1 tablespoon butter on cut sides and toast in a skillet over medium heat until golden, about 1–2 minutes.

Spread about 1–1.5 tablespoons of tartar sauce on the bottom half of each toasted bun. Place a portion of the crunchy cabbage and arugula mixture on top of the sauce (about 3/4 cup per sandwich). Set a blackened fish fillet on the greens, top with 2–3 slices of tomato, and then a handful of firestick onions for crunch. Finish with the top bun. Press gently to compress and serve immediately.