There is a decent variety of restaurants in Nouakchott with plates from 1000 to 2500 UM. Most restaurants in the capital offer much the same menu - simple pizzas, hamburgers, sandwiches and salads. There is a string of restaurants on the road from the Stade Olympique to the French Embassy. Good ones include Pizza Lina, Cafe Liban, and Le Petit Cafe. The Sahara Cafe, on the other side of the stadium, is also a good place for pizza, sandwiches or Lebanese dishes, and has some of the best reasonably-priced food in town. Near Marche Capitale, there is a street of sandwich shops that offer near-identical menus, the best of which is the Prince which taxi drivers know by name.
Outside of Nouakchott, it is possible to find a hamburger in Atar. Otherwise, you are looking at local dishes: fish and rice chebujin in the south and rice and meat or couscous in the north. Hole-in-the-wall restaurants can be found everywhere and serve meals from 200 to 500 UM. Mechui, or grilled sheep, is also delicious if a little more expensive. Look for carcasses hanging by the side of the road. Some fruit can be found in most regional capitals. Note that most restaurants outside of Nouakchott do not have very high standards of cleanliness. Since most small restaurants go under within a few years of opening, your best bet in trying to find one in a regional capital is to just ask locals for directions to whatever is nearby. Another alternative, in the absence of a restaurant, is paying a family to prepare food for you, which should be relatively inexpensive no more than 1500 um, even if it takes a while up to a couple hours to buy the food and prepare it.
Bottled water can be bought for 200 UM and is a good idea for anyone not accustomed to Africa.
If none of this sounds good, keep in mind that boutiques everywhere sell bread, cakes, biscuits and drinks if nothing else!
Tea is usually served after a meal, but it is not included with the meal at restaurants. If you are offered tea in someone's home, it is impolite to leave until at least the second of three glasses. The whole process takes about an hour.