Is it permissible to eat fish meals (Filet-O-Fish) from McDonald's restaurants, knowing that they are prepared in the same place and by the same people who prepare pork meals?

The answer is derived from the predominant assumption regarding the purity of the food. If it is predominantly assumed that pork is cooked in the same pot without washing, then it is obligatory to refrain from eating fish cooked in it. However, if it is predominantly assumed that the fish is cooked elsewhere or in the same pot after it has been purified, then it is permissible to eat it. If there is doubt about washing the pot after cooking pork in it, one must refrain from eating. The Prophet ﷺ forbade eating from the vessels of the People of the Book in which they drink wine and eat pork, unless they were washed. If you do not know or have no predominant assumption, the default for food is permissibility, and scrupulousness is commendable.