250g new potatoes, peeled, cooked and halved, or quartered if very large
2 bunches spring onions, sliced
250g Montgomery cheddar, diced
200ml double cream
Sift the flour and a pinch of salt into a large bowl. Add the butter and rub in until it resembles breadcrumbs. Stir in the icing sugar, egg yolk and 3 tbsp cold water, then quickly bring together to form a firm dough. Wrap well in cling film and allow to rest for 20 mins in the fridge.
Heat oven to 200C/180C fan/gas 6. Roll out the pastry on a lightly floured surface and line a deep 23cm tart tin. Cover with baking parchment and fill with baking beans. Cook for 20 mins, then remove the paper and beans and cook for 5-10 mins more until the tart case is pale and sandy in texture (see p14 for tips on making and baking pastry).
Remove and turn the oven down to 160C/140C fan/gas 2. Spread the base of the tart with the potatoes, onions and cheddar. Whisk together the eggs, milk and cream, then season. Pour into the tart case and cook for 40-45 mins or until just set. Remove, cool on a wire rack then serve.