Mindfulness isn't "thinking about chopping onions while chopping onions." Mindfulness is chopping onions.

The confusion is that "developing" mindfulness means recognizing when, while chopping onions, you're instead thinking about work, or that embarrassing moment from years ago.

There are meditation practices, like "noting", that have you think or say what you're doing while you're doing it. But this is an exercise to train mindfulness, not mindfulness itself.