To make the pastry, place the flour in a bowl and
rub in the butter or margarine until the mixture
resembles breadcrumbs. Mix in the salt and work the
mashed potato into this mixture, adding the milk a
little at a time.
Knead on a floured board until the dough is smooth
and fairly soft. Rollout the pastry and use it to
line a large, shallow ovenproof dish. Line the
pastry case with grease proof paper and fill with
baking beans or dried beans.
Bake in a preheated
oven, 200°C
(400°F), Gas Mark 6, for 10 minutes. Lift out the paper and beans and
bake for a further 5 minutes or until it is light
golden brown.
Meanwhile, cook the mixed vegetables in salted
boiling water until just tender and allow to cool.
Heat half of the oil in a frying pan, add the
mushrooms, fry lightly and allow to cool.
Add the
remaining oil to the pan, fry the onions lightly and
allow to cool. Mix the cooled vegetables into the
white sauce and season to taste with salt and
pepper.
Remove the pastry from the oven, allow to cool a
little and then fill with the vegetable mixture,
spreading with a palette knife so that it is smooth
and flat.
Sprinkle with the grated Cheddar. Brush
the edges of the pastry with the beaten egg yolk and
return to the oven for 15 minutes or until the
cheese is melted and beginning to brown. Serve hot.