How to Make It
Arrange the swordfish in a shallow dish and season with salt and pepper. Rub the garlic onto both sides of the steaks, then drizzle the lemon juice and olive oil over the fish. Cover and refrigerate for up to 1 hour.
In a saucepan, heat the oil. Add the garlic, onion and lemon zest and cook over moderately high heat until lightly browned, 4 minutes. Stir in the brown sugar, lemon juice, vinegar, capers, allspice, ginger and mustard and boil for 3 minutes. Remove from the heat.
Light a charcoal grill. Oil the grate and grill the tomatoes over high heat, turning frequently, until lightly charred. Core and finely chop the tomatoes, then stir them into the ketchup. Simmer over low heat, stirring occasionally, until thickened, about 10 minutes. Season with salt and pepper.
Grill the swordfish steaks over high heat for 5 minutes, shifting the steaks after 2 minutes to create a crosshatch pattern, if desired. Turn the swordfish over and grill for 3 minutes longer. Transfer to plates and serve with the grilled-tomato ketchup.