Step 1Crush the fennel seeds, pepper and one-half teaspoon kosher salt in a mortar and pestle or a spice grinder. Reserve one-half teaspoon of the seasoning salt for the potatoes. Sprinkle the remaining mixture liberally on both sides of the swordfish steaks and set them aside.
Step 2Peel the potatoes and cut them into half-inch cubes. Rinse them well under running water and then steam them in a covered pan over boiling water until they are tender, about 15 to 20 minutes.
Step 3Heat the broiler. Rub the swordfish lightly with one-half teaspoon of olive oil on each side. Broil until the meat flakes when pierced with a knife, about 10 minutes.
Step 4When the potatoes are tender enough that they fracture easily when pierced with a knife, transfer them to a mixing bowl, but keep the water boiling. Beat in the butter until the mixture is light and creamy. Beat in 2 tablespoons of olive oil and the garlic. If the mixture starts to cool, place the mixing bowl over the boiling water to warm, like in a double boiler.
Step 5Off heat, fold in the diced tomatoes, chopped olives and parsley leaves. Add the remaining seasoning salt. Taste and add more salt if necessary.
Step 6Divide the swordfish evenly among six heated plates and spoon a small portion of the mashed potatoes alongside. Serve immediately.