Michell actually had second thoughts about his revenge on Adam.

If you watch closely, when the kitchen thinks the Michelin reviewers are in the restaurant, Adam instructs Michell to do something. But Michell actually looks at Adam like he's concerned or is pondering something. He actually looks like he cares for Adam's well being. Whether it's because Adam just got beat up by the debt collectors or because he has something to tell him, you can see the emotions in Michell's face. It doesn't look like he's all set out for revenge at that point.

But then it all turns. When Michell tries to tell Adam something, Adam has one of his usual outbursts and yells at Michell to do his *beep* job. That's when Michell's expression changes and we see him casually attend to his duties.

I think it's clear that Michell was either having second thoughts about having his revenge on Adam. Or that he wasn't intending to set revenge on Adam in the first place. And the only reason he did was because in that split second, he saw that Adam hadn't changed and was still his usual selfish self.

This is probably the more likely case since I don't think Michell would have worked in Adam's kitchen for months for revenge. Imagine if the Michelin crtics came on a day he wasn't there. The revenge just doesn't make much sense. I think it was something that was done in the heat of the moment.