Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan and add the beef, a few cubes at a time, until all the pieces have been added to the pan. Fry over a high heat until just beginning to brown, then add the onion and fry, stirring, for 5 minutes.
Stir in the garlic and flour, then gradually mix in the wine and stock. Add the tomato purée and bay leaves and season with salt and pepper. Bring to the boil, then transfer the mixture to the slow cooker pot. Cover with the lid and cook on low for 7–9 hours.
Stir the stew, then add the carrots, replace the lid and cook on high for 30–45 minutes.
Meanwhile, make the dumplings. Mix the flour, suet, horseradish, chives and salt and pepper in a bowl. Stir in enough water to make a soft but not sticky dough. With floured hands, shape into 8 balls.
Stir the leeks into the stew, then add the dumplings, replace the lid and cook for another 30–45 minutes still on high or until the dumplings are light and fluffy. Spoon into shallow dishes and serve, remembering to remove the bay leaves.