Preheat the slow cooker if necessary; see the manufacturer's instructions. Make the sauce. Heat the butter and oil in a large frying pan, add the onion and fry for 5 minutes. Stir in the flour, then mix in the stock, port, mustard and tomato purée. Season with salt and pepper, bring to the boil, stirring, then take off the heat.
Make the pastry. Mix together the flour, salt, suet and rosemary. Gradually add enough cold water to mix to a soft but not sticky dough. Knead lightly, then roll out on a floured surface to a circle 33 cm (13 inches) across. Cut a quarter segment from the circle of pastry and reserve.
Lift the remaining pastry into an oiled 1.25 litre (2¼ pint) pudding basin and bring the cut edges together, overlapping them slightly so that the basin is completely lined with pastry, then press them together to seal. Layer the sauce, mushrooms and chestnuts into the basin, finishing with the sauce.
Pat the reserved pastry into a circle the same size as the top of the basin. Dampen the edges of the pastry in the basin with a little water and press the lid in place. Cover with oiled foil and dome the foil slightly. Tie in place with string, then put into the slow cooker pot.
Pour boiling water into the slow cooker pot so that it comes halfway up the sides of the basin. Cover and cook on high for 5–6 hours.