I don't know of any info. from Roman times about what rations were handed out to troops, although I dare say there are some experts who do. We know that Caesar's troops were usually on the move, so my assumption is that they did what armies usually did then and for the next 1800+ years: live off the land. So I would suppose that in Belgia, they enjoyed endives (or the ancient equivalent), cabbage in Germania (maybe sauercrautum ?). After they invaded Britannia, perhaps they used the native crops and plants to create bufo in foramine and Ricardellus maculosus?