A cartouche, in cooking, is a parchment paper lid. The idea is to let a little steam out and keep a little steam in, and these babies do this better than any metal or glass lid.
Start with a square of parchment paper and fold it in half, loose ends to the right.
Fold in half again, loose ends up.
Notice that the folded side is towards me! Very important detail. Now, fold up the bottom folded edge at an angle.
Fold in half diagonally again!
Measure from the center of your pot.
Mark the edge by folding it over.
Cut to size.
Voila! You have made a cartouche. You can now have the tenderest of tender delicious vegetables and the moistest of meats. You also now know an obscure French word.