Remove the venison from the fridge at least 1 hour before cooking. Preheat the oven to 200°C/400°F/gas 6. Crush the juniper berries in a pestle and mortar until fine. Pick the leaves from 3 sprigs of thyme and all the rosemary, then finely chop together. Place the venison on a board and rub all over with 1 teaspoon of olive oil, sprinkle with sea salt and black pepper, then scatter over the juniper and herbs, and give everything a good rub. Preheat a large frying pan on a high heat and sear the venison for 2 minutes on all sides, turning with tongs, then remove to a plate. For the filling, clean the mushrooms and roughly tear any larger ones. Place the porcini in a small bowl, just cover with boiling water and leave to soak for a few minutes. Peel and finely chop the garlic. Wipe the frying pan clean, then add the garlic and mushrooms with the butter and a lug of olive oil. Strip in the remaining thyme leaves, then roughly chop and add the soaked porcini and its soaking liquid (straining to remove any grit). Cook on a medium-low heat for 15 minutes, or until the liquid has gone and the mushrooms are soft, stirring regularly. Tip the contents of the pan onto a board, drizzle with ½ a teaspoon of truffle oil and a few gratings of nutmeg, then roughly chop to a coarse pâté-like consistency with a sharp knife (or blitz in a food processor). Taste and season to perfection, then leave to cool. On a flour-dusted surface, roll out the pastry to 30cm x 40cm. With one of the longer edges in front of you, spread the mushroom pâté over the pastry, leaving a 3cm border. Beat the egg and use to brush the edges. Sit the venison on the mushroom pâté then, starting with the edge nearest to you, snugly fold and roll the pastry around the venison, pushing it away from you and cupping each end to shape it around the fillet. Press the ends together to seal, then indent with the back of a fork. Transfer the Wellington to a large baking tray lined with greaseproof paper, and brush all over with egg wash (you can prep to this stage and chill until needed – just remove it 1 hour before cooking so it’s not fridge-cold). When you’re ready to cook, heat the tray on the hob on a medium heat for a couple of minutes to start crisping up the base, then transfer to the oven for 30 minutes for blushing, juicy venison. Once cooked, leave the Wellington to rest for 5 minutes, then slice. Delicious served with gravy and steamed greens.