Make the ragu by peeling and chopping the onion, carrot and celery. Then heat 1 tbsp oil in a large saucepan and cook the vegetables over a medium heat until soft and lightly coloured. Tip the vegetables onto a plate then add another tablespoon of oil and brown the venison on all over – you may have to do this in two batches. Pour the vegetables back into the pan with the venison and add the wine, passata, 200ml of water, bay leaves, tomato puree and thyme and season with salt and pepper.
Leave the meat to simmer on a medium to low heat for around and hour until it has reduced and thickened. Meanwhile peel and grate the potatoes and celeriac into a large bowl. Season with salt and pepper and pour over the melted butter and mix in the chopped sage.
Heat the oven to 190C/375F/Gas 5 (170C fan). Pour the venison mixture into a pie dish and top with the celeriac and potato rosti and cook for 35-40 minutes until the topping is cooked and golden.
Remove from the oven and serve with seasonal greens.