Siman 464

Mustard, which they call "senf," and it is very sharp and strong, and they dip meat in it, and it contains vinegar or other types of acidity, and they also put a little water in it, and it is the practice to put a little flour in it—it is forbidden to do so on Pesach. For even though the vinegar and that which comes out of the mustard are fruit juice, nevertheless, since there is water as well, it is fruit juice with water and is forbidden, as I wrote in siman 462. And if he did put it in, he should eat it immediately, before it has time to become chametz. Just as we ruled regarding kneading that he should bake it immediately, here too he should eat it immediately.

In what case are these matters stated, that it is permitted to eat it immediately? Regarding mustard, which is sharp and strong, and will not allow it to ferment quickly, and in the meantime, he will eat it. But other types of dips, in such a case where they put flour into them—they are completely forbidden, since they are not so sharp, and it is necessary to linger a bit during the eating, and perhaps in the meantime it will ferment. Therefore, there is no remedy for this, and he is obligated to burn it or cast it into a filthy place. And even if it should appear to him that this dip is as strong as mustard, nevertheless it is forbidden, for we have only that which the sages said (Mishnah 39b).

And this is according to the law. However, it has already been explained in siman 453 that we do not eat types of kitniyot. And mustard is also a type of kitniyot; therefore, we do not eat mustard on Pesach. And even though it was explained there that it is permitted by way of a mixture, and in mustard there is indeed a mixture, nevertheless, the primary element is the mustard. And it is forbidden to eat it on Pesach even if he prepared and arranged it before Pesach, and such is the custom.