Apologize to the customer to show that you understand the mistake you made and that you acknowledge their complaint. Then, find a solution to the problem. In this case, you can take back the salad with cheese (unless they want to keep it to give to someone else). Make a new salad with no cheese and give it to them. Make sure that they're satisfied with their new salad by following up with them. You can also send them a hand-written note with a gift card or some kind of exclusive offer for them (e.g., a discount on their next order, free item(s), etc.) to show your sincerity.