In culinary terms, a cartouche is a round piece of parchment or grease-proof paper that covers the surface of a stew, soup, stock or sauce to reduce evaporation, to prevent a skin from forming and/or to keep components submerged.
Tip: to make one, before you start cooking, tear off a piece of grease-proof paper from the roll which is slightly bigger than you need. Turn the pan you're going to be cooking in upside down on the paper. Draw round it with a pencil (be aware that this then comes into contact with your food, so use something else if you are worried about it) and then cut out with kitchen scissors. If you're really confident, miss out the stage with the pencil and cut with the tip of a very sharp knife. (But do it on a chopping board and not the worktop or kitchen/dinner table!!)
If you realise you need one AFTER you have started the recipe, use the saucepan lid. No lid, then you'll have to guess or use another object (e.g. dinner plate) with similar diameter.
This technique can also be used for lining the bases of cake tins before pouring in the mixture and baking. (To do the sides of cake tin, hold the edge of the tin against the side of the paper and mark with pencil/cut with knife. Either two 2 or 3 short ones and overlap, or go in the same direction as the paper roll to get the length you need.
Another way to make a cartouche is by tearing a small/large (depending on how big you want it) piece of grease-proof paper, folding it in half, then at the folded edge, hold the paper in the middle and fold it in half again. You should then have a small square shape, keeping hold of the.. what should now be a corner.. fold it again into a right angled triangle (you should be holding the same corner all the way through this process. Fold the paper one more time into a thinner triangle, when you have done this, hold the other end of the triangle and hover the thin corner of the triangle (that you were holding before)above the centre of the container you are trying to cover. once you have the correct measurement, tear or cut the end of the paper that you are holding (the ruffly bit) and open the paper back up. You should now have a round-ish piece of greaseproof paper.