Question: I’ve always prepared my egg and liver before Shabbos lunch. Is that a problem?
Answer: The Shulchan Aruch (OC 321:9) writes that the melacha of tochen, grinding, doesn’t apply to food that doesn’t grow from the ground such as meat and eggs. Thus, while one needs to be careful not to chop the onion too fine, one is allowed to cut liver and eggs into very small pieces or mash it with a fork (Shemiras Shabbos Kehilchasa 8:23). The Mishna Berura (321:36) writes, however, that one mustn’t use a specialized utensil such as a grater to cut such foods, as it is uvda dechol, weekday activity. R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 6:3) writes that one may use an egg slicer even for vegetables, as it is simply a few ’knives’ placed closely together.
As shelling the eggs is considered borer, separating, one must only prepare them right before the meal.
Another potential issue with mixing the ingredients together is losh, kneading. R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 8:23) writes that while there are reasons to permit mixing the ingredients together normally, ideally one should do it slightly differently, placing the ingredients in a different order to normal and mixing it in a crisscross rather than circular fashion, etc. (See Mishna Berura 321:68; Baer Moshe 6:44).
In conclusion, while onion and other vegetables can’t be chopped too fine, egg and liver may be chopped and mashed. When mixed with other ingredients, ideally it should be done with slight variations. It should only be prepared right before use.