Preheat the oven to 180C/170C fan/gas mark 4.
Heat half the oil in a casserole and cook the onions over a medium heat until pale gold and slightly soft – about 10 minutes. Turn up the heat, add the celery and cook briskly for a further five minutes or so, stirring often.
In a frying-pan, fry the lardons in the rest of the oil over a medium heat until golden all over. Add to the onions, leaving behind the fat in the frying-pan.
Cook the mushrooms briskly in the frying-pan to get a good colour. Add to the casserole with the garlic and juniper and cook for two minutes.
Pat the venison dry with kitchen paper and add this to the pan with the flour. Cook for a further two minutes, then add the bay, rosemary, nutmeg and plenty of seasoning, followed by the stock and wine.
Bring to a simmer, cover with a lid and put into the oven for 2½ hours, stirring from time to time. If the mixture still has quite a lot of liquor at the end of the cooking time, strain it off into a separate pan and boil it to reduce. Pour it back on the venison and transfer the whole lot to a flat-rimmed 2-litre pie dish.
Turn the oven up to 200C/190C fan/gas mark 6.
Roll out the pastry on a lightly floured surface to the thickness of a pound coin. Cut a strip of pastry the width and diameter of the lip of the dish. Moisten the lip with water and press on the strip. Brush with water, then add the pastry top, pressing to seal. Trim any excess pastry. Knock up the edges by holding a knife horizontally and gently tapping the pastry edge all the way around. Crimp the edges.
If you like, use the rest of the pastry to decorate the top. Make a few small slits or a hole in the middle for steam to escape. Brush with the beaten egg and bake for 40 minutes, until the pastry is puffed up and golden.
Meanwhile, make the buttermilk mash. Cook the potatoes in boiling, lightly salted water until tender, then drain. Return to the pan and cover with a clean tea towel and the lid. Place over a very low heat for a couple of minutes to dry out a bit. Mash thoroughly, or push the mixture through the fine mesh of a Mouli-légumes, sieve or potato-ricer.
Add the butter and some seasoning. Heat the milks and cream, then gradually beat into the potatoes, mixing well with a wooden spoon. Season or add more butter to taste.
Serve the pie and mash immediately.