“Occupe-toi de tes oignons”, literally “Take care of your onions”, means “It’s none of your business”. It’s a very familiar expression used when someone is meddling in someone else’s affairs/business.
Its origin is not very clear. It might come from rural parts of France, when women were sometimes allowed to use a corner of the garden to grow onions that they could sell at the Farmer’s Market for their own profit. The injunction Occupe-toi de tes oignons might then have been a way for husbands to put their wife back in their place if they felt they were interfering with their own business.