Finely chop one of the onions and combine with the pork mince in large mixing bowl. Mash with the hands to integrate well. Form into meatballs and arrange on a greased baking tray.
Roast in the preheated oven for 30 minutes. Remove from the oven.
Preheat the slow cooker to Low setting.
Roughly chop the second onion then place together with the other vegetables, apples, sage, garlic, chicken stock and gravy granules in the preheated slow cooker. Carefully add in the meatballs and give a gentle stir. Cook on low 6 to 7 hours until the meatballs are no longer pink in the centre and the fruit and veg are tender.
An hour or so before you want to eat, remove the meatballs and blend the sauce and veg in a food processor or using a handheld liquidiser (I kept some veg back for texture) then put meatballs back and stir. Add the quark and tarragon and cook for another hour or so then serve with mash or rice.