The secret's in the sauce...hee hee hee!
The employer owns all of the ingredients. They're his. If the chef leaves or is fired, the employer still has all of the things that he needs to make a sauce.
It just won't be this chef's sauce. The way he blends them together, his talent, God gave that gift to him. That's what makes this chef valuable, his God-given talent. The sauce is nothing without him.
It's his sauce, but the employer's ingredients...so I say the chef has all the right to keep the knowledge - but he should return all the ingredients and get his own if he leaves.
He can't take the ingredients when he leaves, so the employer can't take the sauce.
So the secret really is in the sauce, hah!