Aruch HaShulchan, Orach Chayim, Chapter 473, Paragraph 17 states: "If Passover falls on Shabbat, one must prepare the charoset from the day before, as on Shabbat it is forbidden due to resembling kneading. If one did not prepare it from the day before, they should put the liquid into it, but not mix it with a spoon, and one should also give the liquid beforehand." Indeed, the Mishnah Berurah (there: 47) is lenient in this matter, and its language is: "He should make it thick – and he should do it from before Yom Tov, and if he forgot, it is permissible to do it on Yom Tov, see there."