Remove the outer cabbage leaves and set aside 12 large, undamaged leaves. Finely slice the remainder, reserving the trimmings.
Bring a large pot of salted water to a boil with the vinegar and blanch the sliced cabbage. Drain but do not refresh.
Peel and core the apples and cut them into large dice, reserving the peel and trimmings. Toss the diced apple with the lemon juice to prevent discolouration.
Bring a large pot of salted water to a boil, add the cabbage and apple trimmings, and bring to a boil again to make a broth. Cook for about 30 minutes, then strain the liquid and set it aside.
Preheat the oven to 325°F (160°C/Gas mark 3).
In a large ovenproof sauté pan, melt the butter over medium heat and sweat the diced apple until tender. Add the sliced cabbage and pour in the brown ale and the broth from the trimmings. Add the thyme and bay leaf. Cover with a lid, transfer to the oven, and cook for 35 minutes.
Lift out the cabbage and apples with a slotted spoon and set aside.
Place the pan on the stovetop and boil until the juices are reduced and concentrated. Spoon some of the cabbage and apple mixture into the center of each whole cabbage leaf, season with fleur de sel and pepper, tuck in the sides, and roll up into round parcels, using plastic wrap to help give the parcels a neat shape, if you wish.
Remove the plastic wrap, if used, butter a large sauté pan, and place the stuffed cabbage parcels in it.
Add the braising juices and cook over medium heat for 10 minutes, spooning over the juices to glaze the parcels.