Keep venison in marinade made of wine, balsamic vinegar and dried blueberries for at least 3 days.
Reserving marinade, dry venison with kitchen paper and brown each side in a hot frying pan with oil. Move from the pan into a baking dish and bake for 20 minutes in preheated 180 C / Gas 4 oven.
In the meantime put chopped spring onion in the frying pan and fry until golden. Pour marinade over it and let it simmer for about 20 minutes or until reduced in half. Mix well in blender, add cream, then season to taste. Add venison cut in 1cm thick slices and simmer for 1-2 minutes. Remove from the heat.
While the marinade is simmering cook potatoes and leeks cut into smaller pieces. Drain and put back in the pot, add butter and soured cream. Season to taste and mash until smooth.
Form puree balls with an ice cream spoon and serve with few small slices of venison in blueberry and wine sauce.
I actually used cervine leg for this dish but venison is more tender and I recommend it to be the first choice.