As the general manager $-$ What are you going to have$$him "Justin" do first?

Reassure him that, even as bad as it seems, things can get better if he'll change and invite others to change rather than continue to resist changing.

Have Justin apologize to the staff and acknowledge that his mistakes with the menu created a lot of issues with the speed of service and quality of the food.

Invite the staff to start over and pull together as we implement the changes.

Help Justin realize how his lack of leadership and subsequent blaming and defensive attitude in the restaurant was a big contributor to the problems among the staff.