IANAL but I'm fairly confident that would only apply if you are employed as someone who develops recipes. If you are a software developer no court will decide your employer owns the recipe you invented.

If you create the recipe at work maybe (but doubtful). Signing an employment contract isn't signing away your soul to the devil though.

I suspect in this specific case the company could have something to say because it's a competing product,but that would hinge on them arguing that the idea was worked on at work.

Further. Contracts state a number of hours, or at least there are limits to hours worked. How would it go down if you took Friday off because you were thinking about work on Tuesday evening?

This is in the EU, where you have actual employee protections.