Put the shortcrust pastry mix in a large mixing bowl, add the salt and mix together well. Add the margarine and blend until the mixture resembles breadcrumbs. Add the eggs and blend for 20–30 seconds until the mixture comes together to form a sticky dough. If it is too dry, gradually blend in 1–2 tablespoons chilled water. If it is too sticky, add a little extra shortcrust pastry mix.
Shape the pastry into a ball, wrap it in cling film (plastic wrap) and chill in the fridge for 30 minutes.
Put the dried porcini in a small bowl, cover with boiling water and leave to soak for 30 minutes. Drain and chop finely.
Heat the oil in a heavy-based frying pan over a medium heat. Add the onion and cook for 2–3 minutes, stirring occasionally, until softened, then add the garlic and cook, stirring, for another 30 seconds. Add the chestnut (cremini) and porcini mushrooms and cook for about 10 minutes until softened. Season and stir in the wine and the parsley. Cook for 10 minutes more until the wine has been absorbed and the mixture holds together. Remove from the heat and leave to one side to cool.
Season the beef fillet and cook in a hot dry frying pan over a high heat on all sides for 9–10 minutes to seal it. Put two pieces of cling film (plastic wrap) on a clean surface, overlapping slightly. Lay the slices of prosciutto, just overlapping, in two rows on the cling film. Spread half of the mushroom mixture in a horizontal line in the middle of the rows of prosciutto. Sit the beef fillet on top. Spread the remaining mushroom mixture over the top of the fillet. Pull the cling film up on one side and use it to smooth the prosciutto over the top of the mushroom layer. Then pull the other side of the cling film up, smooth the prosciutto over the top and twist the ends of the cling film to tighten everything.
Preheat the oven to 200°C/400°F/gas 6. Dust a large piece of baking parchment with shortcrust pastry mix and roll out the pastry to a rectangle about 4mm/⅛in thick and more than twice the width of the beef fillet. Be careful as the pastry will still be sticky. Remove the cling film and put the beef in the centre of the pastry. Using a sharp knife, neaten the edges of the pastry (you can use any trimmings to decorate the top, if you like). Using the baking parchment to keep the pastry together,
fold one of the long sides of pastry over the beef. Using a pastry brush, brush the edge of the pastry with the egg mixture. Fold the other long side of the pastry over so that the edges overlap slightly. Brush the join with the beaten egg and carefully smooth along the seam with your fingers to secure it. Brush the edges of the two short ends of the pastry with the egg, fold over to seal the sides and smooth the pastry at the seams again. Brush the remaining egg over the top of the pastry, particularly at the seams, and cut three slits in the top. Transfer the parcel to a baking tray. Bake for 25–30 minutes until the pastry is golden brown.
Recipe by Grace Cheetham at https://www.gracecheetham.com/gluten-free-dairy-free-beef-wellington