How to Make
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Preheat the oven to 200ºC/400ºF/gas mark 6 and lightly oil a deep pie dish.
Trim the leeks, leaving 2.5cm of green at the top. Cut into 2.5cm slices and wash thoroughly. Plunge into boiling water or steam for 5 minutes. Drain and place in the base of the pie dish.
Beat the eggs with the cheese and yoghurt and season with salt and pepper. Pour over the leeks.
To make the pastry, mix the flour, oats and salt in a bowl, then rub in the margarine until the mixture resembles breadcrumbs. Add enough cold water to mix to a soft dough. Turn on to a lightly floured surface and roll out to 2.5cm larger than the dish.
Cut off a narrow strip of pastry. Moisten the rim of the pie dish and gently press the strip around the rim. Brush with water, then carefully lift the pastry over the pie dish and place on top. Trim and flute the edges of the pastry with the back of a knife. Roll out the pastry trimmings and use to decorate the top of the pie. Brush with milk or beaten egg to glaze.
Bake in the oven for 30 minutes until golden brown.