Heat oven to Gas 5, 190°C, 375°F. Use a non-stick muffin pan and lightly brush all the cases with a little butter.
Cut the pastry into 18 squares all larger than the pan cases. Lightly brush the pastry on each side with the butter and line the cases, allowing irregular and frilly edges.
Chop the spinach well and mix with the eggs, Parmesan, mustard and crème fraîche.
Mix until smooth, divide between the prepared filo pastry cases and bake in the oven for 12 minutes, until golden and the filling just set.
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