Tip the dried mushrooms, bouillon and vinegar into a bowl and pour over 400ml boiling water. Boil the potatoes and celeriac together for 18-20 mins until very tender, then drain and leave to steam-dry. Mash with the white pepper.
Meanwhile, heat the oil in a large, deep, non-stick frying pan and fry the onions for 10 mins, stirring occasionally, until golden. Add the mince and fresh mushrooms and fry for 5-10 mins until the mince is browned. Tip in the dried mushroom mixture and the sage, then half-cover the pan and simmer for 20 mins, stirring occasionally, until thickened.
Spoon the mixture into four 175ml pie dishes and top with the mash and stilton, if using.
If eating straightaway, heat the grill to high. Put the pies on a baking tray (in case they bubble over) and grill for 5 mins. To make ahead, leave to cool completely, cover and freeze for up to three months. Reheat in the oven until piping hot. Wilt the spinach in a pan or the microwave and cook the peas following pack instructions. Serve the pies with the veg on the side.