Brown the mince in a non-stick frying pan and drain off any fat. Add the stock and tomato purée and season. Bring to the boil, then reduce the heat and simmer for 30-40 mins, adding a little extra stock if necessary.
Mix the cornflour with 2tbsp water to form a smooth paste, then add to the mince and stir until thickened. Transfer the mixture to an ovenproof dish.
While the mince is simmering, boil the potatoes and root veg for around 20 mins, until tender. Drain and mash with the crème fraîche or olive oil.
Pile the mash on top of the mince and cook in the oven for 20 mins, until the top is crisp and golden. Serve with red cabbage and green beans.