Heat the vegetable oil in a non-stick frying pan until hot and add the venison. Fry the meat for a few minutes on each side, until golden-brown all over. Remove the venison from the pan and set aside to keep warm.
In the same frying pan, add the onions, red and green peppers, celery, red chillies, mushrooms and garlic and fry until the vegetables are soft but not coloured.
Add the tomato purée to the pan, then turn down the heat and gently fry for a further ten minutes.
Add the spices, stock, red wine and canned chopped tomatoes to the pan and bring to the boil.
Return the venison to the pan and add the grated chocolate. Reduce the heat to very low and simmer for one hour, or until the venison is tender.
To serve, place the venison chilli onto plates. Spoon some plain rice alongside and serve with the soft flour tortillas.