Hi Rabbi, I accidentally cut onions with my meat knife and sautéed it with spinach. Can I add that to cheese or is it considered meat?
If one cuts a sharp food, such as an onion, with a knife, the food gets the status of the knife. This means, if one cut a sharp food with a meat knife, the sharp food would absorb the taste of meat from the knife, even though neither the knife nor the food was hot. Because the onions are meaty, you should not add cheese to the onions and spinach.
If you already added the cheese, consult a Rabbi.