Tear the mushrooms into a bowl, scatter over 15g of sea salt (most of this will drain away later) and scrunch together, then leave for 30 minutes, scrunching occasionally. Tip the flour into a bowl with a pinch of salt, then chop and rub in the butter. Make a well in the middle, slowly pour in 200ml of cold water, then mix, pat and bring it together. Wrap in clingfilm and chill in the fridge for 1 hour.
After 30 minutes the mushrooms should feel quite soggy, so squeeze firmly to remove as much salty liquid as you can (the mushrooms should end up weighing around 400g). Peel the swede, potatoes and onion and slice into small pieces, roughly the same size and thickness as a pound coin. Mix the veg with the mushrooms, then add the rosemary and a few generous pinches of black pepper.
Preheat the oven to 180°C/350°F/gas 4. Divide the pastry into 8, then roll out into 20cm rounds on a clean flour-dusted surface. Divide up the filling, then scrunch and pile it to one side of the middle, leaving a 2.5cm gap around the edges. Lightly brush the exposed pastry with beaten egg, fold over and press the edges down, then twist and crimp with your fingers and thumbs to seal. Eggwash, then place on a lined baking sheet and bake for 40 minutes, or until golden.
Serve with a watercress and apple salad and a dollop of English mustard.