Slice the sausages into rounds about 1 cm(½ inch) thick. Heat the oil and butter in a large heavy-based pan (cast iron is good) and sauté the onion for a couple of minutes over medium heat. Stir in the paprika, cook for 30 seconds or so and then add the sausages. Continue cooking, stirring fairly often, until the sausages turn golden in places. Add the potatoes, tomatoes, cassia and bay leaf and 500 ml(17 fl oz/2 cups) of hot water. Season with salt and bring to the boil.
Lower the heat, cover and simmer for about 20 minutes until the potatoes are softened and the soup is thick and stewy. Stir with a wooden spoon from time to time and shuffle the bits at the bottom to make sure they don’t stick. If the potatoes are not quite done after that time, take the pot off the heat and leave it with the lid on for the potatoes to continue steaming. Mix the parsley through and serve hot, or even at room temperature.