Pear and blue cheese tart with walnut pastry

shortcrust pastry 500g pack

walnuts

plain flour for dusting

potatoes 150g, peeled and diced

butter 10g, plus a knob more

onion 1, sliced

pear 1, cored and diced

egg

double cream

whole milk

thyme leaves picked to make 1 tbsp, plus extra to decorate

blue cheese or vegetarian alternative (plus some Lancashire and goat’s if you have leftovers) 100g, crumbled