If a chef is fully employed by you, then all that information gained/used in the course of that employment also belongs to you.
If the chef working in your restaurant sells a cake he made there and puts the money in his pocket instead of yours, that's theft. It's legal to sell cakes the chef made, it's just the the resulting value belongs to the employer and not to the chef personally.
If the chef working in your restaurant sells some of the cooking ingredients and puts the money in his pocket instead of yours, that's theft. It's legal to sell spare cooking ingredients from a kitchen, it's just that the resulting value belongs to the employer.
And in the exact same manner, if the chef working in your restaurant sells the customer order statistics (e.g. how many eggs were eaten) and puts the money in his pocket instead of yours, that's theft. It's legal to sell customer order stats, it's just that as far as those stats have value, that value belongs to the employer and not to the chef personally.