Experiencing the sudden escape of aromas when the diner open the parcel at the table is the whole reason of cooking in individual paper or foil parcels. Therefore, you should not attempt to open the parcel to check whether the food has been cooked. Otherwise it would defeat the whole purpose. Choose food that cooks quickly at a high heat, such as fish fillet and finely chopped vegetables and fruit, and stay faithful to the times that are suggested in the recipe.
A small amount of liquid in the parcels also creates a steamy atmosphere that helps to cook the food more quickly. As they are opened, the puffed parcels will release an explosion of appetizing smells and aromas to the delight of the diner.
** Asian Online Recipes