Heat the oven to 200C, Gas 6. Grease a 1.5 litre baking dish.
Keep the filo sheets covered with a clean, damp cloth while you're working, to stop the pastry from drying out.
Lay a sheet of pastry in the dish and brush with melted butter. Repeat with four more filo sheets, brushing with butter as you go.
Heat the oil in a frying pan and cook the mushrooms and onions until soft. Stir in the spinach and cook until just wilted.
Whisk the eggs and milk until combined.
Stir in the spinach mixture, feta, ham and Cheddar and pour into the pastry case.
Fold over any excess filo across the top of the filling and brush the top with butter.
Best served warm and just set.
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