I had the time to try out something I came across a while ago. A half sentence in the Marvel of Milan, a praise poem by Bonvesin de la Riva on the extensive virtues of the city, describes an interesting use for walnuts:
Also almonds, may I say a little about this, wild hazelnuts, walnuts in incredible quantity, which are enjoyed all through the year and all citizens delight in them after any meal/dish (post omnia ferculla).They are also added, ground up, to cheese and eggs and pepper with which meat is filled in wintertime. There is also oil (made) of them which flows richly for us.
This mixture intrigued me, and I got myself some walnut meats and Rouladen beef for rolling up to try a few versions in manageable portion sizes. My first question was what to do about the eggs. Since there are recipes including boiled egg in stuffings for roasts from later years, I opted for hard-boiled egg processed with the nuts and cheese rather than raw to bind the mixture. The next variable to address was the choice of cheese, and I decided to make two stuffings with grated Padano cheese and one with mozzarella to see how they would perform. I do not think either is a very plausible candidate historically, but similarly fresh or aged cheeses would have been available. Processed into a paste with plenty of pepper, they produced a credible filling.
I do not have evidence for this, but for sheer curiosity I wanted to see how this filling would work with green herbs. This turned out to be completely superfluous because the flavour of the cheese and nuts overpowered them almost completely. Something stronger, though, perhaps green garlic, would certainly harmonise well. Given the hostility of the upper classes to anything that made your breath smell, though, I do not think this is likely. The rich, smooth consistency I got very likely was what the recipe aims for.
I spread the stuffing on the Rouladen beef and rolled it up, then cooked them at a low temperature to soften them. The meat turned out pleasantly tender, most likely because of the oil and fat from the cheese and nuts preventing it from drying out. Today, we tend to use bacon for this. It was also pleasantly tast, rich and unctuous. I had added some chestnuts to the mix because Bonvesin de la Riva also mentions them:
Green (fresh), they are cooked in the fire and are eaten after other foods in place of dates, and in my estimation they give a better flavour than dates. They are often boiled and thus softened (? sive lessa) and eaten with spoons by many, thus cooked.
When the water has been discarded after cooking, they are often eaten without bread, or rather in place of bread.
I suspect that when such a roast was prepared on a spit, adding chestnuts to the dripping pan would produce something very similar to what I got. We know this was done with other vegetables by medieval cooks. It is how traditional Yorkshire pudding is still made in a pan under the roast, and medieval Islamic cuisine had an entire class of dishes made this way called judhab. I cannot be sure this is what people did, but it is plausible enough to use until I come across that elusive recipebook from the age.