Light a charcoal grill. In a food processor, combine the leeks, celery, garlic, the 2 cups of basil, parsley and anchovies and pulse until finely chopped. Spoon 1/2 teaspoon of oil in the center of each of four 18-inch foil squares. Set the halibut on top of the leeks and arrange the tomatoes, potatoes and remaining 12 basil leaves around. Season the fish and vegetables with salt and pepper. Spoon 2 tablespoons of water around each fish steak. Bring the sides of the foil up and over the fish, sealing the edges together with short folds.