Photograph: Jan Baldwin
Heat 1 tablespoon of the oil in a wide shallow pan, add the shallots and garlic, and fry gently for 3-4 minutes. Increase the heat, then add the mushrooms, in batches, and fry for 5 minutes until golden. Pour over the brandy and bubble for 2 minutes, then stir in the truffle oil, if using (it gives the mushrooms a more intense flavour). Remove the pan from the heat and stir in the double cream until the mixture is soft but not runny. Transfer to a bowl and leave to cool. Stir in the herbs.
Meanwhile, steam the greens for 3-5 minutes until wilted; drain well. Spread out on a tray and pat dry with kitchen paper. Set aside.
Cut the venison into 8 equal pieces. Heat a nonstick frying pan. Rub each venison fillet with the remaining olive oil, season, then sear the meat for 1 minute on each side in two batches (don't clean out the pan – save the pan juices for making gravy).
Dust a board with flour, then cut each pastry sheet into quarters to make 8 equal rectangles. Roll each piece out to 25 x 16cm. Divide half the greens equally among the 8 pastry rectangles, placing them at one end and leaving an equal border on three sides. Top the greens with a layer of mushrooms, then a venison fillet. Cover each fillet with the remaining greens.
Brush the edges of the pastry with beaten egg. Fold over the pastry and seal the ends and sides to make a parcel, using a fork to crimp the edges. Trim off any excess pastry so you're left with a neat pillow shape. Transfer the wellingtons to a lightly oiled baking sheet. Brush with more beaten egg and sprinkle with the sesame seeds. Chill in the fridge for at least 1 hour before cooking.
While the wellingtons are chilling, make the gravy. Melt the butter into the meat juices from the seared venison, then stir in the plain flour and cook for 1 minute. Add the red wine and beef stock; bring to the boil. Stir in the redcurrant jelly and season. Simmer for 10 minutes or until reduced and thickened. Cover and set aside.
Preheat the oven to 200°C, fan 180°C, gas 6. Bake for 25-30 minutes for medium-rare meat. The pastry should be golden. Reheat the gravy on a low heat, stirring. Once baked, cut each wellington in half and serve with the gravy and roasted root veg.
Get ahead: ideally, prepare the Wellingtons up to the end of step 5 at least 2 hours in advance; chill.