Small amounts of frozen blueberries (common in Northern Europe)
Nuts / seeds (link to article on historic consumption of nuts)
Here is some information on the Sami people. They live amongst the arctic circle in Northern Sweden, Norway, etc and eat almost exclusively animal protein / fat. I'm not of Sami ethnicity, but they are the closest thing to a subsistence culture still living in Northern Europe.
Some foods that I originally thought might have been eaten by indigenous, Northern European living peoples are:
Some form of potato - Most potatoes (sweet potato, etc) are tropically grown and originally from South America.
Parsnip - Originally from Eurasia, brought to Europe through early trade (?). Only can be grown in cold climates, so may still be appropriate if in limited quantities?
Carrot - The modern carrot is from Afghanistan, brought to Europe through trade. Wild carrots in Europe are only edible in the spring and in small amounts. Some small amount of carrot may be acceptable in the diet?
Beets - Traditionally available in Southern to Mid Europe along the Atlantic coast and in Asia. Not historically available to Northern Europe until the introduction of agriculture.I may reconcile the regional discrepancy by including some beet greens (chard).
No please don't take my strawberries and blueberries :-). I have decided to give up nightshades too. I tried eggplant last weekend and I don't really care for it anyway. What else is on that list? I love tomatoes as well, but I'm going to curtail the peppers.