Your goal as a manager is to not only have happy customers but to have satisfied employees too, and you never want to embarrass one of your servers in front of the team or their customers. Employees who feel trusted and competent are more likely to stay with you or the long term.
Discuss how you would ask the server to walk back to the kitchen with the plate without making a scene.
"Every experience like this is an opportunity for further training. If the plate was delivered to the customer, and the customer complained, I could show the server what the plate should look like. I would also use this as an opportunity to further train the kitchen staff on the quality that needs to come out of the kitchen at all times."
"I would never correct a server in front of a customer. Unless you have a policy that states otherwise, I would try to intercept the plate before it arrived at the table. I could use the experience to train further the employee on how to better screen dishes before delivering them to their tables."
"My leadership style tends to be friendly and direct, so I would quietly approach the server to see if the quality of the dish was up to our standards. If needed, we would send the dish back to the kitchen, and I would communicate with the chef to ensure the issue did not arise again."