Sauerkraut. Any kind of fermented vegetable would do but you really need something with relatively high vitamin c content and it’s really easy to destroy.

Fermented cucumbers are used in winter as source of vitamine C all the way from river Oder to Kunming in China. As are other pickles like betrot.

https://en.m.wikipedia.org/wiki/Pickled_cucumber

In warmer climate like Mediteranian children were given wine in winter for that reason.

For Northern Europe at that time I would think that kale would be the best source. You can harvest fresh kale for most of the year. No idea how the vitamin c holds up when you dry it. I'm unsure if sea buckthorn was growing in Northern Europe at that time, but if it was it would be a near perfect source, save for the trouble of picking them. Today you could eat Rosehip too, but that plant is so invasive that it needs to be seriously kept at bay.

Remember that, without understanding the chemistry, running an experiment was difficult. Aside from the expense, I doubt people would be enthused about doing trials of alternative foods when people would get scurvy if the food turned out not to work as well.

> without understanding the chemistry, running an experiment was difficult

Also, without the scientific method, the concept of an experiment could look to an outsider like recklessness (or buffoonery).

Scurvygrass (https://en.wikipedia.org/wiki/Cochlearia_officinalis)

Kept many people alive who had been marooned over the winter in Svalbard and other places in the Arctic. Also grows in cooler climates in Europe

Raw (or lightly-cooked) meat also has a lot of vitamin C.

Yes, sauerkraut.