"The chef used to put butter in all dishes."
Translation:Le chef mettait du beurre dans tous les plats.
For those wondering why « dans » and not « au » or « en », it is because a dish has physical borders. If you said « au », then at a specific dish or a specific spot only. A point could be signified by « en » due to the unspecifity of the dishes, but the fact that dishes do have borders weakens it.
In the given context, "plat" refers not to the container itself but to the food placed in it. Thus the butter is added to the (ingredients of the) dish, i.e., the food that the chef prepared. So it does not mean that a piece of butter is placed on a plate.