Preheat the air fryer to 400°F (200°C) for 3–5 minutes. A hot basket gives you the crispiest edges.

Form the beef into 8 equal balls, about 2 ounces each. Don't overwork the meat.

Smash: Place the balls in the air fryer basket (use a fitted parchment round for easy cleanup) and press each one flat with a spatula or burger press into a thin patty slightly wider than your buns. Work in batches if needed so they aren't crowded.

Season the tops of the patties generously with about 2 teaspoons of the black currant seasoning.

Air fry for 5–6 minutes, until the edges are crispy and the internal temperature reaches 160°F (71°C). Top each patty with a half-slice of cheese during the last minute to melt.

Make the sauce: While the patties cook, stir together the mayonnaise, ketchup, relish, and 1 teaspoon black currant seasoning in a small bowl.

Toast the buns: Butter the cut sides and toast in the air fryer for 1–2 minutes, or in a skillet, until golden.

Assemble: Spread sauce on the bottom buns, add a cheesy patty, then pickles and onion. Cap with the top bun and serve hot.