Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a large frying pan, add the venison a few pieces at a time until all the pieces are in the pan, then fry, stirring, until evenly browned. Scoop the venison out of the pan with a slotted spoon and transfer to the slow cooker pot. Add the onion to the pan and fry for 5 minutes until softened.
Stir in the flour, then mix in the wine and stock. Add the beetroot, redcurrant jelly and tomato purée, then the juniper, 2 sprigs of thyme and the bay leaf. Season with salt and pepper and bring to the boil. Pour the sauce over the venison, cover with the lid and cook on low for 8–10 hours or until tender.
When you are almost ready to serve, preheat the oven to 220°C (425°F), Gas Mark 7. Unroll the pastry and trim the edges to make an oval similar in size to the slow cooker pot. Transfer to an oiled baking sheet, flute the edges and add leaves from the trimmings. Brush with egg, sprinkle with the remaining thyme leaves stripped from the stem and coarse salt and bake in the preheated oven for about 20 minutes until well risen and golden.
Stir the venison and spoon on to plates. Cut the pastry into wedges and place on top of the venison. Serve with roasted parsnips and baby carrots.