Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion, garlic and mushrooms and fry, stirring, for 5 minutes or until just turning golden.
Stir in the flour, then mix in the wine, tomatoes and stock. Add the redcurrant jelly, thyme and salt and pepper and bring to the boil. Pour into the slow cooker pot, cover with the lid and cook on low for 6–8 hours.
When almost ready to serve, make the topping. Put the flour and butter in a bowl, rub in the butter with your fingertips until fine breadcrumbs form. Stir in the walnuts, cheese and salt and pepper. Add half the egg, then mix in enough milk to make a soft dough.
Knead lightly, then roll out the dough on a lightly floured surface until 2 cm (¾ inch) thick. Stamp out 8 rounds with a 6 cm (2½ inch) plain biscuit cutter, re-rolling trimmings as needed. Stir the mushroom casserole, then arrange the scones, slightly overlapping, around the edge of the dish. Cover and cook on high for 45 minutes or until well risen. Lift the pot out of the housing using oven gloves, brush the tops of the scones with the remaining egg and brown under a grill, if liked.