Verjuice is a very common cooking ingredient in Medieval cuisine. When I first encountered cookbooks on the topic, there was still a lot of debate about exactly what it was, although most agreed it was a type of vinegar made with unripe grapes. (It's not fermented and therefore non-alcoholic and not really the same thing as vinegar.) I've known for a while that verjuice was commercially available, but I had never actually seen any, nor, obviously, cooked with it.
This weekend Melisande and I spent the day in Ann Arbor and stopped at a gourmet food and wine shop, Morgan and York (incongruously, it used to be the Big Ten Party Store - party store for the wine and brie set, I would say!). It was a delightful experience, although I was disappointed not to see any artisanal gins I haven't tried yet. Wine was clearly their emphasis. We bought a few odds and ends I hadn't seen elsewhere, along with some chocolate truffles, but the main find from my perspective was a bottle of verjuice. The maker is a South Australian outfit called Maggie Beer. By the way, Morgan and York is one of the pricier shops of its kind I've seen in Michigan. Well worth a visit, though.
I haven't used the verjuice yet. At the price I paid for 375 ml (they are selling it as a gourmet item for salads and sauces like balsamic vinegar), I hope it is useful for medieval dishes. I'll report here when I try it.