Let's return to the Rule of St. Benedict, to see what wisdom it contains about eating.

Chapter 39: Of the Quantity of Food
Making allowance for the infirmities of different persons, we believe that for the daily meal, both at the sixth and the ninth hour, two kinds of cooked food are sufficient at all meals; so that he who perchance cannot eat of one, may make his meal of the other. Let two kinds of cooked food, therefore, be sufficient for all the brethren. And if there be fruit or fresh vegetables, a third may be added. Let a pound of bread be sufficient for the day, whether there be only one meal or both dinner and supper. If they are to eat supper, let a third part of the pound be reserved by the Cellarer and be given at supper.
If, however, the work hath been especially hard, it is left to the discretion and power of the Abbot to add something, if he think fit, barring above all things every excess, that a monk be not overtaken by indigestion. For nothing is so contrary to Christians as excess, as our Lord saith: "See that your hearts be not overcharged with surfeiting" (Lk 21:34).
Let the same quantity of food, however, not be served out to young children but less than to older ones, observing measure in all things.
But let all except the very weak and the sick abstain altogether from eating the flesh of four-footed animals.

Each monk is allotted, per day, one pound of bread (to be split between the two meals on days when two meals are served), and one cooked dish, although fresh fruits and vegetables can also be served when in season. No “meat”, that is, no beef, veal, pork, lamb, or mutton except for the sick and convalescing. The foundation of the daily diet is, naturally enough, bread, the “staff of life.”
What would those cooked dishes have been? Well, pasta wasn’t invented yet, rice was unknown in Italy at this point, and neither potatoes nor corn (maiz for non-US English-speakers) hadn’t been brought back to Europe yet. So dishes could have been made with barley, buckwheat, millet, fava beans, lentils or garbanzos. (Most of our beans such as kidney, pinto, and green beans were also unknown in Europe at the time of Benedict. These all came to Europe, along with peppers, tomatoes, squash and pumpkins, not to mention chocolate, from the Americas after Columbus.)
This diet would have, no doubt, seemed especially bland to modern-day Americans, used as we are to a diet enriched by foods and cuisines from all over the world. But the diet Benedict laid out was one that was sufficient for a person’s daily needs, and even looked out for taste (hence the requirement of two cooked dishes) as well as for the differing needs of the sick and of children.