You can’t take for granted that turnover is imminent in the restaurant business and that you should live with that, that's a defeatist position. There are several alternatives to decrease high rotation but the first is to identify the causes.
When a person who works as a part-time and is promoted to a management position, not only should be promoted by his/her operational knowledge, he/she needs to be trained in management skills. If a bad promotion is carried out, not only will you lose the operational experience that this person has developed, it will also affect the rest of the team.
You can begin by identifying the training and development needs. This evaluation tool includes the reviewing of the selection, hiring, training, development processes, and even an evaluation with the exit interviews.
The strategic human resources plans must have well-established training programs; operational, technical, and personal development.