2) Combine the garlic and the olive oil in a small bowl. Brush both sides of the halibut with the oil and let marinate for 15 minutes.
3) Heat the griddle to high.
4) Season the halibut with salt and pepper, to taste, on both sides and grill until slightly charred and just cooked through, about 5 minutes per side. Remove from the griddle to a platter. Garnish with tarragon and serve immediately with the tomato relish on the side.