Slowly render pancetta until browned on all sides. Add red onion and sliced garlic and sweat until the onion is translucent. Add the red cabbage and season the mixture with salt and pepper. Allow to cook for about five minutes. Add apple cider and allow cabbage to braise slowly for 45 minutes to an hour or until very tender. (If necessary, add more cider keep from sticking during the braise.)
Taste the cabbage for seasoning then quickly deglaze the pan with a splash of red wine vinegar.
Grill each loin on both sides to rare or medium rare, depending on your preference, then remove from the heat and allow the meat to rest.
While the meat is resting, quickly reheat the cabbage mixture. Stir in 2/3 of the chopped chestnuts and warm through. At the last minute, add the fontina cheese and allow to just barely melt.
Combine the apples with the remaining chestnuts and whole leaf parsley. Season with salt, pepper and olive oil.
Slice the venison and place gently atop the cabbage mixture. Place apple salad on top of grilled loin, then serve.