Place all the vegetables in the top of a steamer and lightly steam for about 10 minutes. Saute the onion and garlic in 2 tablespoons of stock for 2 minutes. Add the crushed tomatoes and stock, stir in the tomato paste. Combine the water and cornflour to make a paste. As the gravy begins to bubble, stir in the cornflour paste. Add the parsley and cook for 5 minutes on low heat, stirring continuously. Place the vegetables into a casserole dish and spoon over the gravy. Spread the mashed potato over the vegetables and gravy evenly, using a fork. Place in a water bath and cook at 230°C for 40 minutes.