They dont really care about your order. You say no onions in the burger and you get one with onions. In total, I was there in last 3 years around 10 times, 3 times they got my burger with onions (even if I specify 2 times during my order " do not put onions".
For a burger of 18$ plus tax without any side, you have to improve your service. You lost a client, I gave you a chance.