Apparently, it's a common misspelling. The word here is supposed to be 'desert', pronounced exactly as 'dessert'. It's a now archaic word, meaning something that you deserve to receive, whether reward or punishment. Sources: https://en.wiktionary.org/wiki/just_deserts https://www.merriam-webster.com/dictionary/desert

It's not desserts, it's deserts, though it is pronounced the same, i.e. the stress is on the second syllable, rather than the first as it is in Sahara Desert. Desert here means 'what is deserved', your just deserts are whatever you justly deserve.

Desert - dare sert -- an arid place. Dessert - dee cert -- sweet food after the savory dishes are served. 1. "as a verb" "desert" is pronounced the same as "deSSERT" means to abandon. 2. https://www.dailywritingtips.com/just-deserts-vs-just-desserts/