Dampening the cartouche helps to maintain a moist environment, e.g. sweating onions. Dry cartouches are used too, when baking pastry blind.
1. To make a cartouche, cut a square or round greaseproof paper and fold into segments, the shape of an elongated triangle. Trim the triangle to a fraction larger than the radius of your pan.
2. Unfold the paper into a circle that should fit snugly inside the saucepan on top of the onions.
3. Crumple the cartouche and dampen under cold water before use. The crumpling helps the greaseproof paper hold a little more water. Lay on top of the ingredient(s) to be cooked.