I recently bought this skillet from Lidl. I seasoned it by heating it with oil, salt and an onion. It started looking like this after the first time I used it, I cleaned it with a rag, tried washing it but couldn't get rid of the burnt look. The second time I used it, the food that was in the middle of the skillet got burnt really easy. I have no idea how to properly clean this skillet. Any ideas?