Preheat the oven to 200 degrees celsius, fan oven 180 degrees celsius, Gas Mark 6
Sift the flour and salt into a large bowl. Rub in the butter and vegetable fat until the mixture looks like breadcrumbs. Add 2-3tbsp cold water to make a soft (not sticky) dough. Knead lightly until smooth. Wrap and chill for 10 minutes.
Drain the salmon, break into chunks and set aside. Melt the butter in a frying pan. Gently fry the spring onions and asparagus for 5 minutes. Cool.
Roll out the pastry on a lightly floured surface and use to line a 20cm flan tin. Line with crumpled foil and bake 'blind' (i.e. without a filling) for 15 minutes. Reduce the oven temperature to 180 degrees celsius, fan oven 160 degrees celsius, Gas Mark 4.
Remove the foil and fill the flan with the salmon chunks, spring onions and asparagus. Beat the eggs and creme fraiche together, season, then pour into the flan. Bake for 30-35 minutes until set. Cool.