An amazing thing happened when I watched Chef Isao Yamada simmer ingredients in the Japanese way, a technique called nimono. When the simmering liquid started to boil, he laid a lid, a wooden lid smaller than the saucepan, directly on top of the cooking ingredients. The liquid immediately started boiling up, but then Yamada-san adjusted the heat, and it calmed down, happily bubbling away under that lid. What was going on here?
That lid is called an otoshibuta, or drop lid, and it's a genius piece of Japanese kitchen equipment. As I said, it's smaller than the diameter of the saucepan, so it fits inside, rather than covering it. As Yamada-san explained, the otoshibuta performs three primary functions: First, it circulates and increases heat, so the ingredients cook quickly and evenly. The cooking liquid also circulates towards the lid, so it coats the top of the ingredients inside the saucepan without having to stir or spoon. Second, the increased temperature caramelizes the sugars in the cooking liquid, adding flavor, and evaporates and concentrates the cooking liquid. And third, the otoshibuta holds ingredients in place so they stay stable, don't move around and don't break apart.
If you're using a wooden otoshibuta make sure to soak it in water first. If it's dry it will soak up the cooking liquid, which you don't want, of course. There are plastic or silicon otoshibuta, too, plus you can easily fashion one out of aluminum foil or parchment (I'll explain in by doing this in a future post). In fact, for some delicate ingredients like kabocha or turnips, you want to use parchment instead of a wooden lid, which would be too heavy and crush the fragile ingredients once they start simmering. Finally, when using an otoshibuta, make sure to keep a close eye on your pot -- things cook fast and you'll burn your food if you don't pay attention. (And guess who's done that?)