Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the meat a few pieces at a time until all the pieces are in the pan, then add the onion and fry over a medium heat, stirring until the meat is evenly browned.
Stir in the flour, then gradually mix in the Guinness and the stock. Stir in the horseradish, tomato purée and a little salt and pepper, then add the bay leaf and bring to the boil. Transfer to the slow cooker pot and press the meat below the surface of the liquid. Cover with the lid and cook on low for 8–10 hours or until the meat is cooked through and very tender.
When almost ready to serve, preheat the oven to 200°C (400°F), Gas Mark 6. Discard the bay leaf and divide the beef mixture between 4 pie dishes, each about 450 ml (¾ pint). Mix in the mushrooms, then brush the top edge of the dishes with a little egg. Cut the pastry into 4 and roll each piece out on a floured surface until a little larger than the dishes, then press on to the dishes. Trim off the excess pastry and crimp the edges. Mark diagonal lines on top and brush with beaten egg. Put on an oiled baking sheet and cook in the preheated oven for 30 minutes or until golden. Sprinkle with the Stilton and leave to melt for 1–2 minutes. Serve with green beans and curly kale.