Trim the meat and cut into 1-inch chunks and put into a bowl. Add the port, garlic and thyme, mix well and leave to marinate overnight or for at least 4 hours.
Preheat the oven to 180°C/Gas 4.
Drain the meat and pat with kitchen roll to remove excess moisture. Dust the venison with flour and heat the oil in a frying pan and sear the venison in batches until they are all caramelized.
Put the celery, carrots and onion in a roasting tray and place the seared meat on top. Pour the red wine, chicken stock and the marinating juices over the meat. Add the pearl onions, diced celeriac, chestnuts and mushrooms and cook for about 2 hours or until the meat is tender. Remove from the oven and stir in the gewürztraminer and chopped flat parsley.
To make the rosettes, place the potatoes in a pot with cold water and bring to a boil, reduce heat and simmer for 4-5 minutes until the potatoes are par-boiled. Allow to cool and cut into even sized slices. On some baking parchments, make a circle of overlapping potato slices about 6-inches in diameter and brush with the melted butter. Roast in the oven for about 10 minutes until potatoes are tender but not browned.
Divide the venison mixture into individual 6-inch oven-proof dishes and place the cooked potato rosettes carefully on top and place in the pre-heated oven for 15-20 minutes until they are crispy and golden on top and serve immediately.