To make the relish, heat a grill to moderately high. Lay the garlic scapes and scallions directly on the clean grill grates and grill them, turning occasionally until they soften and become charred in spots, about 8 minutes. Keep the grill on. Chop the scapes and scallions and transfer them to a bowl. Stir in the celery, olives, mint, lemon juice and olive oil. Season with ¼ teaspoon salt and several grinds of black pepper. Let the relish sit at room temperature while you grill the fish.
Lay the fish on a baking sheet and stuff the cavity of the fish with the scallions, lemon slice and mint. Season the fish well, inside and out, with salt and pepper and drizzle with olive oil. Lightly oil the grill grates, then lay the fish directly on the grates and grill, covered, for 6 to 8 minutes. Using a wide spatula, carefully flip the fish and continue to cook, covered, until the flesh is flaky, about 6 to 8 minutes more. Transfer the fish to a serving platter and spoon some of the relish over the fish.
Printed with permission from Burgess Lea Press, an imprint of the Quarto Group.