Start by making the garnish. Wash and dry the grapefruit, then pair off thin strips of the rind, using a citrus zester. Scrape off any pith that remains attached to the strips. Cut the grapefruit in half. Squeeze the juice from the half of the grapefruit into a small bowl, add the strips of paired rind and leave to marinate for 1 hour. Cut the other half of grapefruit into wedges and reserve