The Shulhan Aruch rules that it is permitted to chop cooked meat on Shabbat. This is based on the Terumat HaDeshen, who restricts the prohibition of Tohen-Grinding only to "Geduleh Karka"-foods that grow from the earth, such as grain. Meat is not considered to be from the earth, even though the animal was raised and grew from feed that grew from the ground. The Mishna Berura explains that there is no need to rule in accordance with the strict opinions that consider an animal as Geduleh Karka, since the opinion of the Rosh, who holds that there is no prohibition of grinding any type of food, can also be used as a basis for leniency regarding animals. Therefore it is permitted to chop meat or an egg, even well in advance of the meal.
The Rema brings the principle "En Tehina Ahar Tehina"-there is no prohibition of grinding substances that have already been ground. For example, it is permitted to crumble pieces of bread into crumbs, since the flour that made that bread was already ground. Hacham Ovadia brings an additional application-crumbling snuff. Sometimes the tobacco becomes clumped to together due to the humidity, but there is no problem to rub it into granules, since the tobacco leaves were already ground. The same would apply to banging a salt shaker to re-pulverize the salt that has clumped.
SUMMARY
It is permitted to chop meat or an egg on Shabbat, even well in advance of the meal.
It is permitted to crumble bread into crumbs or separate clumps of snuff and salt.
