For most of society, grain and lots of it. In, The Making of Europe Robert Bartlett traces the gradual expansion of Latin Christendom in medieval Europe. He does this by following the spread of bishoprics across Europe. He notices that as Latin Christianity spread, so too did a cereal monoculture.

It should be said that this does not mean that people ate exclusively bread. Indeed, during the high Middle Ages, medieval farmers realized that certain crops replaced the nutrients that wheat stripped from the soil. Cereals, especially wheat are incredibly hard on soil and required crop rotation in the premodern period.

Although potatoes revolutionized European diets, this reliance on wheat and bread remained strong well into the modern period. Steven Kaplan has written extensively on the grain supply of Paris during the age of the ancien régime.

If you have further questions about this, feel free to ask.

Earlier foods included wild leaves, berries, hazelnuts, mushrooms. Grains such as wheat and barley, were boiled in a pot over the fire, or made into griddled oatcakes. Onions, cabbages, root veg and herbs were also added. Once you had oats you could mix them with butter or honey. Lets not forget birds and eggs, or the dairy, especially in pasture-rich Ireland. With cows milk you could have butter, fresh curds, cheese, sour cream. On the coast seaweed made a nice dish. Then came the potato, which easily took over as staple food since you could have big crops with only a small plot of land. The potato eventually killed the variety of the Irish diet as it was easy to grow and easy to cook. A lot of farmers started growing only potatoes. When the famine left them without any income, they couldn't buy any other foods to feed themselves.

further reads: "Food and Drink in Britain" by Anne Wilson "Land of Milk and Honey: The Story of Traditional Irish Food and Drink" by Brid Mahon

I asked a question about food in Northern Europe before potatoes. It might help