Rissani is not a place to come for the food. There are a handful of cheap restaurants around the souk and the bus station — tagine, brochettes, harira soup, bread — and almost nothing else. Prices are low even by Moroccan standards and the cooking is the kind you would get in any other small Tafilalt town.
The town's one genuine speciality is madfouna, sometimes called Berber pizza: a thick round of bread stuffed with spiced minced lamb or beef, onions, almonds, and spices, baked in the embers of a wood oven. Ask for it at any of the small restaurants in the medina — it takes 30 to 40 minutes to cook, so order it as you arrive and have a glass of tea while you wait.
For anything beyond the basics, eat at your riad or kasbah-style guesthouse. The small family-run places south of town toward Merzouga are where the best home cooking in the area happens — fresh bread, slow-cooked tagines, and dates in every conceivable form.