The shift in what was consumed commonly throughout Medieval Europe cane in late antiquity and early Medieval ages, as it shifted from meats and dairy products to more wheats, fruits and vegetables.
Christianity on Food
As Christianity became the dominant religion, via its traditions and various practices restricted to the populace what it would consume during Lent and Advent, permitting only wheat, fruits, vegetables and fish to be consumed during such times. Yet it seems the term fish was loosely based, as it included beavers, ducks, whales and puffins among others. The practice of Lent was not to deter or to prohibit people from eating certain types of food due to their class or due to being considered unclean, rather to teach them how to restrain themselves and to sacrifice their body in order to strengthen their souls just like Jesus did for humanity. Even though the imposed rules were strict, people could manage to find a way around those practices. They compensated later in the form of a penance. With evidence and sources we find that foods consumed during the Medieval ages were not plain or only restricted to certain cooking methods.
Cooking
Cooking actually required someone to be a master of foods cooking was done upon an open fire, as opposed to stoves, which were scarce and required large quarters to house them. Thus bread was not restricted to private consumption. Wild and domestic animals were the staple source for meat of the mainland, while those near water usually consumed fish and any other aquatic animals.
With the introduction of spices and herbs via the Silk Road as well as bringing back the vegetables like potatoes, rice and tomatoes from over-seas allowed them to be spread all around Europe. Certain foods were then restricted, like wheat, fruits wine and other artisanal products uncommon in the north.
The south on the other hand, lacked the diversity of wild game as well as specific vegetables, spices and herbs. As diseases ravaged the world, it forced people to adapt and to survive off of preserved foods meant to be consumed during harsh times or extreme weathers. Thus, smoking, drying, brining and fermenting were introduced and practiced. They were brought back from antiquity and allowed for the creation of methods that produced artisanal foods like butters, cheeses, buttermilks, beers, wines, sweets and desserts.
With time progressing, it brought with itself more and more favorable living conditions for everyone, from the flora, fauna and people alike, it also allowed for more ways for people to prepare their foods. That resulted in the expansion of recipes, and what was once considered exotic, became available and more frequent.