Answering your question, to Dr. `Abd al-Fattah Idris, professor of Islamic jurisprudence in Al-Azhar University, said,
If you know that the meat or any other food that is originally halal for Muslims is cooked with fats or oils extracted from pigs, then it is haram (prohibited) to eat this food. Similarly, if the food is cooked where pork was cooked, e.g. in the same pot or pan, it becomes haram to eat it, because the impurity and defilement of the pork will then be transferred to the food.
So, it is better for you to eat other kinds of food that are far from being cooked this way, such as roasted fish, vegetables or the like, if you have to go to these restaurants. There are many other restaurants that are owned by Muslims and provide halal food and you should go to such restaurants instead.