Heat the oil in a large flame-proof casserole on the hob and fry the garlic and onion for about 5 minutes until beginning to brown. Stir in the chilli powder and cook for a further minute.
Add the potatoes, turnip, leeks and mushrooms to the casserole and cook on the hob for 3 or 4 minutes. Stir in the flour and cook for another minute.
Gradually stir in the stock with the tomatoes, tomato purée, thyme and season with plenty of salt and pepper. Bring to the boil, stirring, Cover and put into the oven for 30 minutes.
Whilst it is in the oven, make the topping by rubbing the butter into the flour, stir in half of the cheese with the chives and plenty of salt and pepper. Add just enough milk to make a smooth dough.
Roll out the dough into a circle until it is about 1 inch (2.5 cm) thick. Cut into 12 triangles and brush with milk.
Remove the casserole from the oven, add the beans and stir to combine. Place the triangles on top and sprinkle with the remaining cheese.
Return to the oven, uncovered, for 20 to 25 minutes until the topping is golden.
Serve with two cheesy triangles per person. Enough for 6 people.