Preheat the oven to 350 degrees F and butter a large, shallow baking dish.
Slice the potatoes and celeriac into disks just under 1-inch thick. Place the slices into a large pan, cover with cold water, season with salt and pepper and bring to the boil. Simmer for 5 minutes, then drain in a colander and allow the vegetables to steam dry for a minute or so. Put back into the pan with the cream, chopped garlic, sage, half the Parmesan and a good pinch of salt and pepper. Mix together, then tip into the buttered baking dish and spread out evenly. Pour any mixture left in the pan over the top. Sprinkle over the remaining Parmesan, cover tightly with aluminum foil and cook in the preheated oven for 35 to 40 minutes until golden brown.
Combing the juniper berries and rosemary, add a pinch of salt and pepper, then sprinkle over a board. Rub the venison all over with olive oil before rolling it across the board and pressing it into the flavorings. Heat an ovenproof frying pan over a high heat and add a glug of olive oil. Sear the venison for a couple of minutes on all sides, then remove the pan from the heat. Add the smashed garlic bulb and any leftover flavorings from the chopping board. Shake everything together, pour in a splash of water to cool things down and place in the oven. Cook according to your liking - about 8 minutes will give you medium venison.
When the potatoes are cooked, take them out of the oven, remove the foil and sprinkle over the remaining Parmesan. Return the dish to the oven, uncovered, and bake for another 10 to 15 minutes until bubbling and golden.
Take the venison out of the oven and let it rest on a plate, covered loosely with foil. Pour away any excess fat. Squash the garlic cloves with a fork and discard the skins. Mix the garlic with the herbs in the pan and place on the heat. Pour in the red wine, simmer until it has reduced by half and then add the butter. Stir with a wooden spoon, scraping up all the sticky meaty goodness from the bottom. As soon as the sauce comes together, take the pan off the heat, correct the seasoning and stir in another knob of butter. Carve the venison into desired-thickness. Pour any resting juices from the plate back into the pan, then pour your gravy through a sieve over the meat and serve with the potato and celeriac bake.
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