Soak the cedar plank in cold water for at least 2 hours. Combine the mayonnaise, pesto and lemon juice in a small bowl. Store in the fridge. Preheat your barbecue to medium-high. Remove the plank from the water and dry one side. Lay a slice of prosciutto on a work surface and set a piece of halibut at one end. Brush the fish lightly with the oil; season with pepper. Wrap the prosciutto around the fish and set on the plank. Repeat with the remaining fish. Top each piece of fish with a basil leaf and a slice of lemon. Place the plank on one side of the barbecue. Turn the heat below the plank to low; leave the other side set at medium-high. Close the lid and cook the fish for 15 to 18 minutes, or until just cooked through. Keep a spray bottle of water handy just in case the board ignites. Serve the fish with a dollop of the pesto mayonnaise alongside.