Combine tomatoes, olives, onion, parsley, tarragon and extra-virgin olive oil in a small bowl and season with salt and pepper, to taste. Let sit at room temperature for 30 minutes before serving.
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.
Heat the grill to high.
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 grill to a platter. Garnish with tarragon and serve immediately with the tomato relish on the side.