Melting cheese and onion pie

onions 3-4 large

butter

floury potatoes 400g, peeled, halved and cut into ½ cm slices

lancashire cheese 200g, grated

mature cheddar 50g, grated

double cream

plain flour 150g, plus extra for dusting

self-raising flour

sea salt

butter 150g, chilled and diced

white wine vinegar

egg 1, beaten