On Monday I posted a 6 question quiz about medieval cooking. I had tried to phrase the questions so that there would be many possible answers that could be considered to be correct depending on viewpoint. Here are my thoughts on the first question.
1. What process would you use for converting a modern recipe into a medieval one?
One answer to this is to replace all ingredients not available in medieval Europe with similar ingredients that were available, and for an extra measure you could replace any modern cooking methods or equipment with medieval ones that achieve similar results. The problem is that this doesn't get you a medieval recipe. It gets you a variation of a modern recipe.
The classic example of this is the cheeseburger. In medieval Europe they had almost all the ingredients and equipment necessary to make a cheeseburger. They didn't have tomatoes or ketchup, but they did have mustard and even had what they needed to make mayonnaise. The problem is that they didn't make sandwiches, they don't seem to have served raw vegetables (lettuce, onion, pickles) with meats, and they didn't make mayonnaise.
So even if you grind the beef in a mortar, cook it on a grill over an open fire, put it on a home-made bun, top it with home made cheese and heirloom lettuce and onion slices and pickles, and use camaline sauce instead of ketchup, what you end up with is still a cheeseburger. It may be a very nice cheeseburger, but it's still not even remotely medieval.
In short, you can't convert a modern recipe into a medieval one. Imagine trying to convert a Mexican dish into a Thai one. The best you can hope for is something cooked in a Thai style.
Take a typical recipe for burritos, replace the cumin and garlic with ginger and lemongrass, serve it with soy sauce instead of salsa, and you've got a Thai-style burrito (beef or chicken - I don't think it'd work with a bean burrito). Is it a real Thai recipe? No, not really.
Of course your goal may not be to make a medieval dish. You might be trying to avoid new-world ingredients, or experiment with new flavors. But then it wouldn't be a question of converting a modern dish into a medieval one. It'd be more one of incorporating aspects of medieval cuisine into a modern recipe.
So if you want to make a medieval recipe, then start with a medieval recipe. If you want to be creative in the kitchen and create a new recipe, go right ahead. You can even combine the two - but the results aren't necessarily medieval cuisine.