I know that wheat, barley, rye, onions, lettuce-like plants and turnips were common but how common were other things like meat or cheese? I've heard the "plowman's lunch" was a concoction of the dairy council so that's out as an approximation. I'd assume that a fair amount of poor quality beer or wine figured into the diet as well.
Here is one noting:
The European medieval diet was largely determined by social class. For the majority of the of the people, peasants, a large portion of their daily diet was made up of grains such as wheat, rye, oats or barley(carbohydrates). The grains were boiled whole in a soup or stew, ground into flour and made into bread, or malted and brewed into ale. Estimates from the late Middle Ages indicated that a gallon of ale a day was not unusual, but the actual alcohol in the drink was low. Protein was usually provided legumes such as beans, peas or lentils, fish where available, or on very rare occasions, meat such as poultry, pork, or beef. Additional nutrients were provided by seasonal vegetables and fruits. The peasant's diet rates high on modern nutrition standards. But seasonal fluctuations in food availability and poor harvests often caused long periods of very poor nutrition.
From Jeffrey L. Singman, Daily Life in Medieval Europe, Westport, Connecticut: Greenwood Press, 1999, P. 54 - 55.
To add another datum, from a later period but probably relevant nevertheless, Henry IV justly prided himself very much on the fact that under his administration every peasant family could afford a chicken meal every Sunday. This represented a very high living standard for peasants.
Obviously this varies considerably by location as well as occupation and social standing - I'm afraid 'peasant' covers a wide array of people.
I'm more familiar with the English diet than anything on the continent, but by far the bulk of their sustenance came in the form of pottage. Basically throw whatever green things you are currently getting from the garden into a pot and leave it on the fire for 3 hours to cook (it prevented food borne illness - a little overkill, but they didn't exactly have germ theory). This sounds horribly monotonous just always eating pottage, but this conceals the wide variety of plants which would be coming available in the gardens as different herbs and vegetables come into season (I suspect the winter and early spring pottage did get a little boring as this was whatever stored well for months - lots of peas and beans and tubers).
After that, much of their calories came from ale. Lots and lots of ale - during the day this would have been small ale, so the alcohol content was quite low (but after the day's work was done, strong ale kept you happy).
One of the ways England was considerably different than the continent would be the presence of a lot of sheep. The typical peasant didn't eat much meat or fish, but the sheep produced plenty of milk, which was turned into cheese. This 'white meat' was peasant food because it was cheap and plentiful (compared to actual meat).
The occasional bit of goose, pig, lamb (needed the rennet for making cheese), or salt-water fish if near the coast (fresh-water fish were too valuable), were occasional treats but hardly a staple of the diet for the lowest farmer. Obviously the wealthier you were, the more of that you ate.