First off, go visit Ref's page: http://vikingfoodguy.com/wordpress/
Second, remember to think critically about things like food storage and preservation. https://sites.google.com/site/kadasresearch/food-preservation/fermented-vegetables
Now, let's talk about hypotheses.
A hypothesis is an educated, proposed explanation. It requires the collection of facts and evidence before a hypothesis can be made. After a hypothesis is made, it should be tested, to see if, in practice, it still matches up with all the available evidence.
How does that relate to what people in Viking Age Scandinavia ate? Because we don't know, but we can make some delicious hypotheses :)
Where do we start? We know a fair amount about the available cooking implements for VA Scandinavia, and a decent amount about the available ingredients. Here are some good starting places:
What we DON'T know is how they put these things together. That part is where they hypothesis comes in. None of the recipes listed here are something we can PROVE that Viking Age people ate. All we can say is that the ingredients and methods are appropriate, and that it tastes good to us. Which may be a problem in itself, as Viking Age Scandinavian folks mostly ate fish and milk, which is not the bulk of an average modern diet. For me, this is the exciting part. The part that means I get to play with ingredients, and try things, and eat lots of delicious experiments.
I'll try to include a brief rationale for each recipe, but no promises that will work for every single one.
I should also note that I am not an "amounts" cook. I am not a baker. All of these recipes will consist of "some", added to a "pinch", and made until it is "right". Sorry about that. I know that it is awkward for other cooks to recreate what I have done, but I don't measure stuff, sigh. Plus, what tastes good to you is different that what tastes good to me, so you will have to experiment and figure out a way to make it delicious for you.
There are a lot of fancy recipes around the internet (including here!). I seriously doubt that is how Viking Age people cooked.
We know that the primary sources of protein for Viking Age people were fish and milk. They were not particularly malnourished, which means a varied diet. Grinding grain by hand is slow and annoying, bread was not a staple. Most of the extant cooking gear consists of pots.
Go to the sources, and make a shopping list of ingredients appropriate to your time period and area (and season!). Go get them.
Put a pot of water on to boil. To that pot, add some whole grain, whatever vegetables and seasonings from your pile of ingredients sounds delicious to you. and then add either fish or cheese. Occasional flour, meat, and fruit is all great.
For example, today I ate boiled oats with shreds of kale, garlic flowers, and sour cream. Because those are the things that are available right NOW.
If I did the same process in the winter, I might eat boiled wheat soup with smoked cod and fermented carrots, and in another bowl, rehydrated dried apple and blackberry leaf tea.
Pick some things. Put them in the pot. Eat it.
Stop worrying about if you are following a recipe ;)
If you want to read the primary liturature, I've pinned most of it here. It is mixed in with modern recipes I use for inspiration, and images of both historical and recreated food, cooking devices, dishes, etc.
This page has some amazing resources, but you need to be smart about using them. This is a compilation of many archaeological reports. The reports are from many sources, including middens (trash heaps), cooking pots, but also pollen analysis. Pollen analysis gives what plants were growing in the area, but says nothing about what of those were used for food. There are many entries in that database that are NOT edible, or were likely not used as food items (such as grass). Be very careful when using this as evidence.
Shelagh Lewins has a bunch of recipes on her site, though her documentable and non-documentable recipes are mixed together.
There are two groups on facebook, the first is a discussion of items, both food items and cooking gear. The second is for recipes.
Here is a photo of some ingredients and equipment we used for some Viking Age possible cooking:
Equipment: Riveted iron pot, staved bucket, ceramics, wooden bowls and boxes, knife, drinking glass, etc.
Ingredients: barley flour, salt, white wine, acorns, plums, eggs, blackberries, onions, chives, sage, hazelnuts (whole and ground), apples, elderberries, dill, carrots, sour cream.