Physicists sometimes refer to "The Arrow of Time" - the apparent directionality of how time flows. Apparently there are some sub-fields of physics where this is a big thing. To those of us outside of that academic world, this seems like a "Duh!" kind of thing, and one wouldn't think it is worth considering in the area of historic research.
When people look at medieval cookbooks, there seems to be a strong desire to ignore the inherent directionality. I think it's part of a built in human need to generalize - to make sense of something that does not fit in with the current world-view.
Ok, that's a bit too philosophical. Let's try some examples.
Let's say you've got this recipe that's been in your family for many generations. One day while serving it to some family or friends the thought hits you that all the ingredients in it were available in medieval Europe. "This might be medieval," you think, and decide to look into it. You then spend months digging through the cookbooks, trying to find recipes with the same ingredients, where similar methods were used, and where the end result sounds similar to the dish you know and love. If you're really really lucky you find an exact match. More likely though is that you find a few "kind-a, sort-a" recipes from widely varying times and places, and then you give up in frustration and tell people that researching medieval recipes is hard.
It's not, really. It only seemed hard because you ignored the directionality of time. You latched on to a piece of modern information and tried to push it some 500 years into the past. You assumed that because all of the ingredients were available in the middle ages, that there must be a cook somewhere back then that made this recipe. [This kind of reasoning is proven invalid by recipes like mayonnaise - all of its ingredients were present back to ancient times, but it wasn't invented until 1756.]
The easier way to research medieval recipes is to keep the information flowing from the past to the present. Pick up (or download) a medieval cookbook and treat it like you would any other cookbook. Read through it, skipping here and there, looking for something interesting, something that sounds tasty or different or that you have all the ingredients called for. If you can't read Middle French or German then use someone else's translation. If you can't read Middle English, borrow (or download) a dictionary and practice - it's mostly funny spelling and a handful of archaic words.
So you've picked out a recipe. Now gather the ingredients and follow the instructions. It may take a couple of tries (or three, or four) before you get the quantities balanced so the flavor and consistency are so you like it - most medieval recipes inconveniently leave out any measurements). You may even find some recipes that sound good but turn out just plain nasty (I've done this, but I've also found such recipes in modern cookbooks). Once you've found a good recipe though, then you've got a truly authentic medieval dish, and you've also got the original source it came from - documenting it becomes a trivial matter.
Of course every now and then, while browsing through a medieval cookbook, you're reading a recipe and the realization hits you, "That's grandma's recipe!" - and you find out that in medieval times they put raisins in it.