Eating options in Atar are basic but adequate. A handful of small restaurants and canteens in the market area serve the standard Mauritanian diet: rice with fish or meat, mechoui (roasted lamb), and thick bean soups. The food is consistently filling if rarely surprising.
Tea is the social glue of Mauritanian life. The three-glass ceremony — progressively sweeter, served from a height to create froth — is offered everywhere and refusing it is considered impolite. Accept and allow time for the ritual; it is one of the more pleasant ways to spend an afternoon in the Sahara. Fresh dates from the palm groves around town are excellent and cheap.
Do bring some provisions from Nouakchott if you have specific requirements. Fresh vegetables and fruit in Atar are seasonal and limited to what comes up from the coast.