Boil the potatoes in boiling, salted water for 15-20 minutes until soft, adding the parsnips after 10 minutes. Drain and mash, adding the butter and enough milk to make a soft mash. Add the mace with salt and pepper to taste.
Meanwhile, heat the oil in a large frying pan, add the onion and juniper berries and cook over a low heat, stirring often, for 10-12 minutes until the onion is soft and translucent. Remove to a bowl.
Add the venison to the pan and cook over a medium heat for about 10 minutes, stirring until evenly browned and in granules.
Heat the oven to 200°C (400°F, gas mark 6). Return the onion with the mushrooms to the frying pan. Add the port and cook over a medium heat for 2 minutes, then add the stock and bring to the boil. Cover and simmer for 15 minutes.
Mix the cornflour, Worcestershire sauce and 2 tablespoons cold water in a cup. Add to the venison and stir over a low heat for 1 minute. Add salt and pepper to taste.
Spoon the venison mixture into four small ovenproof dishes, making sure the liquid is evenly distributed. Spread the potato and parsnip mash on top and fork the top.
Bake for 25-30 minutes until the top is brown and the juices are beginning to bubble up around the sides. Serve with curly kale or cabbage.
This pie freezes well - simply assemble the pie and freeze before baking. Once you're ready to bake, simply throw it in the oven straight from the freezer.