Draw a 27cm diameter pencil mark on a board or platter. Draw a 16cm diameter circle in the centre to form a wreath shape. Trim 1mm from one end of each tomato so they stand upright.
Beat the cream cheese until slightly softened. Stir in the olives and pesto, season.
Arrange the crackers, 2-3 deep, slightly overlapping in the wreath shape. Spread each cracker with thick layer of pesto cheese mixture. Arrange the tomatoes, cut side down over the crackers. Poke the large basil leaves under the crackers.
For the basil oil; blend the oil and basil until smooth. Just before serving drizzle over the wreath. Sprinkle with finely grated pecorino if desired.