Rosso has no restaurant scene to speak of. Food options are limited to a handful of small local eateries near the market and the ferry landing that serve simple, filling meals — rice with fish sauce (thiéboudienne), grilled meat with bread, and sweet tea. These places cater to travellers in transit and local workers, not visitors with time to spare.
If you are waiting for the ferry or a bush taxi, buy food from the market stalls or the women who sell bread and cold drinks around the landing. Stock up on water before crossing in either direction — the border area can involve long waits in open sun, and hydration matters in a climate this hot.