The town of Waterloo has a solid range of restaurants along the Chaussée de Bruxelles and around the town centre. The battlefield complex itself has a café at the Memorial 1815 visitor centre for a quick lunch, but better food is found in town.
Belgian staples — moules-frites, carbonnade flamande, steak-frites — are well represented. A friterie (Belgian chip shop) is an essential stop: frites come with a bewildering range of sauces, and a mitraillette — a baguette stuffed with meat, frites, and sauce — is the battlefield worker’s lunch of choice. Brussels is 20km away by train if you want a wider choice for dinner.