Preheat oven to 120°C.
In a large casserole pot, bring verjuice to a gentle boil. Add sugar and stir to dissolve. Spoon Glühwein Gewürz into infuser bag and seal as recommended. Place infuser bag and vanilla bean in the verjuice and allow to simmer, while you prepare the quinces.
Peeling quinces is optional, but visually more appealing when serving. If peeling, keep the peel and add to the pot as well as it adds to the richness of colour.
Carefully halve, core and thinly slice (about 5mm thick) the quinces, popping the slices straight into the casserole pot as you go to prevent browning.
Making sure all quince slices are covered in liquid – add some extra verjuice or water if not – cover with lid and place in the oven for about 5 hours, or until soft and a rich ruby colour.
Remove infuser bag, extra peel and vanilla bean before serving.
