Put the dried mushrooms in a heatproof bowl and pour over 500ml boiling water from the kettle. Stir and press them down so they’re well covered, then leave to soak.
Heat the olive oil in a wide frying pan over a medium heat, and fry the onion and chestnut mushrooms until the veg is very soft and the mushrooms look wilted and reduced, about 20 mins.
Stir in the garlic and sweet potatoes, then the rehydrated mushrooms and their soaking liquid, leaving the last dregs of the liquid in the bowl (this may contain grit from the mushrooms). Cover and cook for 15 mins until the sweet potatoes are just softened, and most of the liquid has evaporated. Add a splash more water halfway through if the liquid evaporates too quickly.
Remove the lid and stir in the chopped herbs, breadcrumbs, molasses, nuts and cranberries. Season well and sprinkle in the nutmeg. The mixture should be stiff enough to resemble stuffing, but not dry – all the breadcrumbs should be coated and have absorbed some of the liquid. Remove from the heat and leave to cool in the pan.
Once cool, unroll one of the puff pastry sheets onto a lightly floured non-stick baking sheet or baking tray lined with baking parchment. Whisk the milk and mustard together with a pinch of salt and set aside.
Spoon the cooled filling down the middle of the pastry sheet in a neat line, leaving a 5cm border around the edges. Use the mustard-milk mix to brush the borders, then top with the second pastry sheet. Press the pastry around the filling to remove any air pockets, then crimp to seal the sides. Trim off any excess pastry with a sharp knife and use a fork or your fingers to make a pattern around the edge of the wellington.
Cut a small steam hole in the top of the middle, then brush the top with the mustard-milk mix. Chill on the sheet or tray for 30 mins. Will keep in the fridge for up to two days.
Heat the oven to 200C/180C fan/gas 6. Remove the wellington from the fridge and brush with more of the mustard-milk mix. Score a pattern into the top of the pastry with a cutlery knife or fork, if you like, then bake for 35-40 mins until golden, puffed up and crisp – cover with foil if it’s browning too quickly. Arrange the whole sage leaves on top of the wellington and stick them down with the mustard-milk mix, then return to the oven for 10 mins more.
Leave to rest on a board or platter for 10 mins before cutting into chunky slices to serve.