Rub butter and flour together to fine breadcrumbs and stir through walnuts.
Add egg yolk and enough water to mix to firm dough. Wrap in plastic and refrigerate for 10 mins.
Roll pastry to fit four mini quiche tins (loose bottomed)
Press into flutes. Line with greaseproof paper and add baking beans.
Bake for 10-15 minutes, remove paper and beans and back again for 5-10 minutes.
Filling
Heat oil in heavy-based frying pan. Cook onion for 3-4 minutes until soft. Add spinach, stirring until just combined. Stir in blue cheese. Spread cheese mix evenly over cooked pastry bases.
In a large measuring jug, whisk together eggs, milk, and seasoning.
Pour over the cheese mixture in each pastry case and sprinkle with parmesan cheese.