Given that food preservation techniques were not perfect, and many Fall harvest foods perished (i.e. apples) or were unavailable in the winter (i.e. dairy and eggs and anything else that grows), what would the average rural worker in Northern Europe eat in Spring and early Summer when Nature was not providing much in the way of food stuffs, particularly as much work needs to be done in these seasons so caloric intake would have to be high? What would be some typical meals?
For some more detail: I can see how wheat flour is easily stored and full of calories, but what about a population reliant at this time on potatoes? Potatoes store, but can readily perish in a matter if months if there are any problems with storage. Where would people get their calories in early summer if their potatoes rotted?
Salted and dried edibles were common, as well as fermented foodstuffs. The oldest known fermentation plant is found in southern Sweden and is 9,000 years old. Interestingly, Sweden had a salt shortage between 1520-1530 because of Gustav Vasa, which meant that the production of surströmming (fermented fish, yes, that fermented fish) increased. The same thing happened in 1716-1720 because of a conflict with Britain. Before the potatoes arrived, root vegetables were very common. I don't know much about the history of smoking for preservation, unfortunately.
There are apple varieties that can be stored until late winter if you're lucky, but I remember reading that hardy apple varieties weren't introduced in most of Finland until the 18th or 19th century, so we can assume that most fruit that was eaten in the winter had probably been dried, and that the northern parts of northern Europe didn't have a lot of fruit available at all.
In any case, for Sweden, you would probably be looking at a diet consisting mostly of preserved meats, root vegetables, onions, peas (introduced in the 1500s), fava beans (there have been findings in the Nordic countries from the Bronze age) and grains. (Chili seeds from the 13th century has been found in the south of Sweden, but it doesn't mean that chili was a common food.) All could be preserved, stored as-is or easily dried and rehydrated. It needs to be noted that even a small country such as Sweden has very different climates and growing conditions, especially if you compare the north and south. People in the south would have an earlier spring and less risk of starvation, whereas in the north of Sweden, you would be more reliant on meat and have smaller margins before you could harvest. And if the harvest failed, or the stored foods somehow perished, you were at a real risk of starvation and death. Bark bread was not an uncommon staple in those situations.