Put the mushrooms in a food processor along with the onion, garlic and thyme. Pulse to a coarse paste, then transfer to a nonstick pan and cook for 12-15 mins, stirring occasionally, until it turns a rich, dark colour. Set aside and allow to cool completely.
Cut off ⅓ of the pastry and refrigerate or freeze to use another time. This should leave you with a rectangle about 26cm x 18cm rectangle. Brush with some of the beaten egg, then spread the mushroom mixture over the pastry in an even layer, leaving a 5cm border all the way around.
Arrange the cured chicken down the centre on top of the mushroom mixture, placed end to end to form one longer cylindrical piece.
Carefully ease up the longer sides of the pastry around the meat, with the edges overlapping. Press together, then carefully roll the Wellington over so the join is on the underside. Crimp each of the open ends shut, so the meat is completely enclosed. Brush with the remaining beaten egg to glaze, then score a decorative pattern in the top with a sharp knife. Chill in the fridge until needed.
Preheat the oven to 200C/180C Fan/Gas 6.
Use a knife to cut ¾ of the way down into the potatoes at 3-5mm intervals. Put them in a bowl, drizzle with the oil, then arrange on a baking sheet with the cut sides up.
Put the Wellington and the potatoes into the oven. Cook for 40-45 mins, or until the pastry and the tops of the potatoes are crisp and golden brown.
Meanwhile, tip the peas and water from the tin into a pan and simmer to warm through. Drain, reserving the liquid, then blitz the peas with a handheld blender until smooth – add a splash of the reserved liquid to thin, if needed. Keep warm.
Heat the spinach in a pan, then drain well.
Divide the spinach, potatoes and pea purée between 4 plates. Slice the Wellington into 4 pieces, then lay on top of the spinach to serve.