How to Make It
Heat the oven to 400°. In a large roasting pan, toss the potatoes with 1/4 cup of the oil, half the garlic, and 1/4 teaspoon each salt and pepper. Put in the oven for 15 minutes; stir once.
Meanwhile, coat the swordfish with the remaining 1/4 cup oil, garlic, 1/4 teaspoon each salt and pepper, and the rosemary. Put the fish on a baking sheet and put it in the oven along with the potatoes. Cook, stirring the potatoes once, until the fish and potatoes are both done, 10 to 12 minutes for 1-inch-thick steaks.
In a small bowl, combine the mayonnaise with the parsley, capers, and vinegar. Serve the roasted swordfish and potatoes with the caper mayonnaise.