Before cooking the tuna, combine the parsley, lemon zest and orange zest in a small bowl. Stir in the finely chopped garlic and reserve.
Cut the tuna into bricks about 3 inches long and 2 inches wide, 3 pieces per portion. Season the tuna with rosemary, salt and pepper, to taste. Drizzle with extra-virgin olive oil and let stand for 10 to 15 minutes. When ready to cook, heat a healthy drizzle of extra-virgin olive oil in a skillet over medium-high heat. When the oil is hot add the tuna and cook for 2 minutes on each side for rare, 3 to 4 minutes on each side for opaque fish. Douse the pan with the lemon juice. Arrange the fish on a platter, scatter the gremolata over the top and serve.