Simon, I have no doubt that you’re still good at what you do, there’s just something getting in the way. Starting a new job and then having the added pressure of knowing that you’re going to be their superior probably amounts to two stress points as opposed to one. Have you been a superior in the kitchen before (lacking the knowledge of proper terminology)?
Take a step back and focus on regaining your confidence. Visualize yourself in the kitchen, see yourself performing confidently and then maybe go back to the knowledge that you possess, you’ll probably find that you have a wealth of it. Relax and tap into it combining your confidence and drawing upon things that you know you’re good it. Don’t allow your posture to slack, force yourself to adopt a quicker step. Even do a workout to get your adrenaline pumping a little bit.
I know you can do it, and you know it, too. Start believing it.