1. Is it permissible to mash food on Shabbos?
2. If one left a pot with water on a Shabbos blech with the lid inverted, is it permissible to place food on top of the inverted lid during Shabbos?
1. Some authorities are lenient to mash soft foods (such as a banana or avocado) with the teeth of a fork (provided it is done for immediate consumption), whereas others are stringent, and require a shinui – for instance, mashing with the back of a spoon.
2. Yet, it is permitted to place food on top of the inverted lid to heat up on Shabbos, provided of course that the food was already fully cooked before Shabbos.
1. According to Rav Moshe Feinstein, it is permitted to mash an avocado or a banana (Iggros Moshe, Orach Chaim 4:74, Tochen 2), if this is done right before the meal.
The reason for this is that no pieces are ground, and because the Rashba rules it is permitted to grind for the purpose of immediate consumption. Also, many maintain that when a food is already fit for consumption, the prohibition of grinding does not apply.
However, the Chazon Ish is stringent in this matter, as is Shemiras Shabbos Kehilchasa (6:1). Yet, all agree that it is permitted to mash the avocado by using the back of a spoon, which is not the regular way of mashing it.
2. Concerning using an upside down pot see Shemiras Shabbos Kehilchasah note 112; Shevet Halevi 1:91; and see also http://www.dinonline.org/2011/02/03/shabbos-hatmana-returning-food-to-heat-on-shabbos/, note 12-13.