Is it permissible to smear food on another food on Shabbat, such as smearing butter on a piece of bread? The Shabbat prohibition of Memareah forbids spreading a solid substance such that it becomes like a liquid. Does this prohibition apply also to foods, such that it would be forbidden to smear food? This question is relevant to many different foods which people spread on other foods, such as mashed potato, avocado and banana.
The Rama (Rabbi Moshe Isserles, Cracow, 1520-1572), in his Darkeh Moshe (Orah Haim 321), writes that it is permissible to spread baked apple on food on Shabbat, and he codifies this ruling in his glosses to the Shulhan Aruch. This ruling is accepted by the Mishna Berura (Rav Yisrael Meir Kagan, 1839-1933). Although in other contexts smearing is prohibited on Shabbat, in this instance it is permissible, since the prohibition does not apply to foods, and in any event the food is perfectly edible even without smearing the other food.
This is also the ruling of Hacham Ovadia Yosef, in Hazon Ovadia (p. 264), though he adds (in the footnotes) that smearing should preferably be done immediately before one eats, as in this case there is no concern whatsoever.
Summary: It is permissible on Shabbat to smear butter and the like (such as mashed potato and avocado) on bread or other foods, though this should preferably be done immediately before the food is eaten.
