Rinse and pat dry the swordfish steaks. Set aside.
In a small bowl microwave melt the butter. Add the olive oil, zest of half the lemon and the juice. Crush and chop the garlic.
Strip the rosemary sprig and discard the stem. Finely dice the three the fresh rosemary, it should come to about 1 tablespoon. Whisk all together and pour over the swordfish. Seal in a container to marinate for an hour, I like to use a Ziploc.
Grill to your liking. I like to get grill marks on mine and 2-3 minutes per side should do the trick. Baste with the marinade.