Heat a pan with some oil over hot heat. Dry the fillets with paper towels. Liberally sprinkle the blackening spice on one side of the fillets and place seasoned-side down in the pan for 3 minutes. Flip the fillets and sear until cooked through, 3 minutes more.
Toast the rolls in melted butter until brown. Spread some Sriracha mayonnaise on the tops and bottoms of the rolls. Season some lettuce with lemon juice and pepper. Place a fillet on each bottom half of the rolls, and add some tapenade, seasoned lettuce and pickled onions, then finish with the top of the roll to serve.
To make caper tapenade, mix equal parts chopped capers, onions, tomatoes, garlic and green olives.