I know that eating fish and meat is a problem. Can I cut onions with a meat knife if cooking/eating it with fish?
Fish and meat that were mixed are forbidden to be eaten because of sakana. However, the Taz (יו”ד סי’ צה ס”ק ג) writes that if the fish merely absorbed the taste of meat from a utensil, the fish is still permitted. Therefore, it is permissible to cut an onion with a meat knife and use the onion for fish.
All the best