The website you want is Medieval Cookery. They've got:
Links to digitized versions of actual medieval cookbooks--some in translation, some in the original languages. These aren't going to be recipes in the modern sense; ones with measurements and such don't start appearing in any appreciable amount until the very late 18C or 19C, and there's a lot of "and add spices" or such. But they're really, really cool to look at, and a good place to start.
Links to online modernized recipes to follow, adapted from medieval outline-versions
I also own the books Pleyn Delit and The Medieval Kitchen, which are modernized versions of medieval recipes.
I hope this gives you some good, uh, food for thought and further reading!
Happy feasting!