Preheat oven to 400°. Mix 4 Tbsp. unsalted butter, room temperature, 2 tsp. finely chopped parsley, ½ tsp. finely grated lemon zest, ½ tsp. freshly ground mixed pepper, and 1 garlic clove, finely chopped, in a small bowl to combine. Season with kosher salt; set aside.

Heat 1 Tbsp. extra-virgin olive oil in a large heavy ovenproof skillet over medium-high. Season four 6-oz. swordfish fillets (1" thick) with salt and mixed pepper. Cook swordfish until browned underneath, about 3 minutes. Turn fillets over and transfer skillet to oven. Roast until swordfish is just cooked through, 8–10 minutes. Transfer to plates.

Set skillet over medium-high heat and add reserved butter mixture. Cook, scraping up any browned bits stuck to the bottom of pan with a wooden spoon until melted and bubbling. Immediately pour butter sauce over swordfish.