Potato and butter pie with mushroom gravy and wilted spinach

Name: Potato and butter pie with mushroom gravy and wilted spinach
Ingredients: 900g/2lb medium potatoes (preferably maris piper); 75g/2½oz fridge-cold butter, cut into 1cm/½in cubes; 1 large onion, sliced into thin rings; Flaked sea salt; Freshly ground black pepper; 350g/12oz plain flour, plus extra for rolling; 175g/6oz butter; 50g/1¾oz extra mature cheddar cheese, finely grated; 1 large free-range egg, beaten with 1 tbsp cold water; Beaten egg, to glaze; 500ml/18fl oz just-boiled water; 2 tsp vegetable bouillon powder (or ½ vegetable stock cube); 10g/¼oz dried mushrooms, either mixed or porcini; 1 tbsp sunflower oil; 15g/½oz butter; 150g/5½oz small chestnut mushrooms, wiped and finely sliced; 1 tbsp plain flour; 2 tbsp finely snipped fresh chives (optional); Large bag young spinach leaves; 25g/1oz butter; Freshly grated nutmeg, to taste; Salt and freshly ground black pepper