What is the ruling on eating fish at a restaurant whose owner's religion is unknown, and which serves meat and chicken that may mix with the fish?

If it is highly probable that chicken meat is not ritually slaughtered (non-dhakī), and fish is cooked in the same unwashed utensils, then it is obligatory to abstain from eating the fish due to its contamination. However, if the fish is cooked elsewhere or in the same utensils after they have been purified, then it is permissible to eat it. If you doubt whether the utensils have been washed after cooking non-dhakī meat, the default assumption is that they have not been washed, and it is obligatory to abstain from eating the fish. The Prophet, peace and blessings be upon him, commanded washing the utensils of the People of the Book before eating in them if a Muslim could not find any others.