Sift the flour into a large bowl, add the butter and rub in with your fingertips until the mixture resembles fine breadcrumbs. Add enough cold water to make a ball. Bring the dough together, wrap in clingfilm and chill for 30 minutes.
Divide the pastry into 4, roll out each piece and use it to line 4 lightly greased tartlet tins, each 9 cm (3½ inches) across. Bake blind (i.e. bake the empty cases without filling) in a preheated oven, 200°C (400°F), Gas Mark 6, for 5 minutes. Then remove the cases, but leave the oven on.
Meanwhile, put the spinach in a large colander, pour over boiling water to wilt the leaves, then squeeze out any liquid. Divide the spinach among the pastry cases. Whisk together the crème fraîche and egg and season well with grated nutmeg and black pepper. Stir through the feta. Pour the mixture into the pastry cases (still in their tartlet tins) and cook for 15–20 minutes or until golden and puffy. Serve with a side salad.