Heat some oil in a large casserole pan and saute the vegetables until golden brown. Add the lamb, yeast extract, stock cube and just enough water to cover. Bring to the boil and simmer for 45 minutes until thickened. Season to taste.
Meanwhile bring 1 litre water to the boil and add the polenta in a steady stream, whisking constantly. Remove from the heat and beat in the oil and parmesan. Pour into a baking tray until it is about 1.5cm thick, then allow to cool and set.
Preheat the oven to 200C. Spoon the lamb and veg into an ovenproof dish. Using a glass or pastry cutter, carve out circles of polenta and alternate slices of polenta and tomato on top. Grate over a little more parmesan and bake for 40 minutes, or until golden brown and bubbling.