In general, you should portion a raw ingredient into its final shape and size prior to packing and cooking. This speeds cooking because the pieces of food are made small; and the amount of work that must be done during service is also reduced, which makes cooking easier.

Another reason to portion prior to cooking is because slicing hot food can be difficult. Even the sharpest blade will tend to stretch and tear delicate foods like this freshly cooked Steelhead trout. Not only does this make the fish look less appealing, but it really slows you down during service.

It's also easiest to thinly slice through a steak or similar meats after they have been cooked. So portion them into individual portions when raw, but do the final slicing just after cooking.

When crispy skin is important, it's always best to do the final portioning and slicing after you've cooked the meat and crisped the skin.

When an ingredient will be served cold, we always prefer to do our final portioning after both the cooking and chilling steps. Cooking inevitably shrinks and warps the shape of an ingredient, so by portioning after cooking and chilling, it's easy to cut identical portions.

Portioning after cooking and chilling an ingredient can also be applied to foods that are destined to be served hot. This approach is especially useful when the appearance of the food is paramount. A large awkward cut of meat, like pork belly, can be cooked sous vide until tender and then chilled. Once cold, the pork belly will be firm enough to easily cut into perfect looking portion-sized blocks. These can be repackaged and reheated later during service.