A cartouche is a circle of greaseproof paper cut to the diameter of your pan and placed directly on the surface of the cooking liquid. When poaching fruit, it keeps all the pieces fully submerged so they cook evenly and absorb the flavoured liquid uniformly. Without it, the top surfaces of the fruit are exposed to the air, cook unevenly, and can even discolour. Scrunch the paper lightly before pressing it down so it conforms to the irregular surface of the fruit. It is a basic but essential tool of the professional kitchen that any home cook can adopt immediately.