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Heat the oven to 200C/180C fan/gas 6. Unravel the pastry on a lightly floured work surface, and sprinkle the 2 tbsp of hard cheese over one half of the pastry along with ½ tsp cayenne and a crack of black pepper. Fold the other half of the pastry over to close it like a book, and roll out, squashing the cheese in the middle, until it fits into a 22cm tart tin with an overhang. Prick the base with a fork, cover with a sheet of parchment and fill with baking beans. Cook for 15 mins, then remove the parchment and beans. Bake for 10-15 mins until lightly golden, then leave to cool in the tin.
Line a baking sheet with baking parchment. Spoon small piles of the remaining hard cheese (about 16) in a thin layer onto the sheet, then bake for 5-7 mins until bubbling and melted. As soon as they look lacy, remove from the oven or they will burn quite quickly. Transfer to a wire rack to cool and crisp up. Will keep in an airtight container for two days.
Put the spinach in a sieve over a clean bowl, and squeeze well with the back of a spoon. Keep the liquid.
Heat the butter in a heavy-based pan and fry the shallots with a pinch of salt for 10 mins. Stir in the garlic, thyme and 2 tbsp flour, then pour in 250ml of the spinach water, boiling for 1-2 mins. Stir in the mascarpone, nutmeg and a pinch of cayenne, and cook for 5 mins.
Fold through the cheddar and spinach, tip into a bowl and leave to cool for 5-10 mins. Stir in the eggs. Sprinkle the breadcrumbs over the tart case, then pour in the filling. Bake for 35-40 mins until golden. Leave to cool a little before topping with thyme and the cheesy crisps.