Dice all the large vegetables into 1/2cm dice.
Season the shanks and fry in a frying pan to give them even colour all over and then place into an oven dish that has a lid.
Fry all the vegetables in the pan you have used for the venison until coloured then add tomato purée and cook for 5-8 min. Add the stock and sage.
Cover the shanks with the stock and vegetables then cook in oven for 1½ hours or until meat is tender. Larger shanks could take slightly longer.
Remove shanks from cooking liquid and keep warm and covered so they do not dry out.
Decant the liquid into a saucepan, passing through a fine strainer to remove vegetables - keep them warm with the shanks.
Bring liquid to the boil, add mustard and reduce by half.
Drop baby beetroots into cold water and bring to boil until cooked then place in cold water, and peel.
Sauté the chestnuts in a 15g of butter to warm through.
Reheat the baby beetroots by plunging in boiling water, then drain and toss in 10g melted butter.
Place the shanks in a serving dish and cover with hot sauce, then garnish with vegetables, baby beetroots and chestnuts.