Whisk together the lemon juice, lemon zest, olive oil, soy sauce, and garlic.
Pat dry the fish then season both sides with salt and pepper. Add the marinade to a ziplock bag and add the swordfish steaks in it. Marinate the fish for 20 minutes.
Heat a non-stick skillet over medium heat. Add the butter and once melted, add the swordfish steaks and cook for 3-4 per side, until fully cooked.
Remove from the heat and brush with melted butter and serve immediately.