Put your oven on 220C to heat up, and get a kettle full of water on to boil. Get your pastry out of the fridge. Put a frying pan over a medium heat.
Put the dried mushrooms in a mug or bowl and cover with boiling water. leave to steep while you do everything else. Finely chop the chestnut mushrooms, and peel and finely chop the red onion (or use a food processor).
Put the lentils in a pan of boiling water and simmer for fifteen minutes or until tender. Drain really well in a sieve, rinsing with cold water to get the temperature right down. Once cool, squish with your hands to squeeze out as much liquid as possible.
Add a little olive oil to the frying pan and add the mushrooms and onions. Pick the leaves off the thyme and add to the pan. Season with salt and pepper. Fry gently for four to five minutes until softened, then turn up the heat and add the wine to deglaze the pan. Fry off until all the moisture has gone, then tip on to a cold plate to cool as quickly as possible.
Wipe out the pan and add the spinach leaves. In a minute or two they will have wilted down. Transfer to a chopping board, roughly chop and mix the spinach through the lentils.
Roll out your pastry sheet on baking paper on a metal baking sheet. You should have it portrait style in front of you. You will only need to work with the top half, as the bottom half will roll over the filling to make the complete sausage shape. As you add filling in the next step, leave a 1cm gap above the mixture to seal.
Brush the top half with Dijon mustard. Spoon over your mushroom and onion duxelle and flatten with the back of a spoon. Drain your dried mushrooms and scatter over the duxelle. Now add your lentil mix, making a rough sausage shape.
Brush the spare 1cm you left with a little water to provide a seal. Gently pull the pastry over the filling and press together to seal. Using a sharp knife make several slits over the top, just piercing the pastry to allow steam to escape. Brush with beaten egg, sprinkle with sea salt and bake. Cook for 25 minutes or until the pastry is completely risen and golden brown. Cut in thick slices and serve with mashed potatoes and gravy (a gravy sweetened with redcurrant jelly works really well with this).