Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a frying pan, add the onions and fry for 5 minutes or until softened. Sprinkle the sugar over the onions and fry for 5 more minutes or until browned. Add the mushrooms and fry for 2–3 minutes. Stir in the flour.
Mix together the stock, mustard, Worcestershire sauce and salt and pepper in a jug.
Make the suet pastry. Put the flour, salt and suet in a bowl and mix well. Gradually stir in enough water to make a soft but not sticky dough. Knead the dough lightly, then roll out on a floured surface to a circle 33 cm (13 inches) across. Cut out a quarter segment and reserve.
Press the remaining dough into a 1.5 litre (2½ pint) buttered pudding basin, butting the edges together.
Layer the fried onions, mushrooms and sliced steak in the basin. Pour the stock over the top. Pat the reserved pastry into a round the same size as the top of the basin. Fold the top edges of the pastry in the basin over the filling, brush with a little water and cover with the pastry lid.
Cover the pudding with a large domed circle of buttered foil so that there is room for the pastry to rise. Tie with string. Stand the basin in the slow cooker pot on top of an upturned saucer. Pour boiling water into the pot to come halfway up the sides of the basin. Cover with the lid and cook on high for 5–6 hours.
Remove the basin from the slow cooker using a tea towel and remove the string and foil. The pastry should have risen and feel dry to the touch.