Grease the tart tin generously with butter.
To make the pastry sift the flour and salt into the mixing bowl or food processor bowl.
Add the butter and lard and rub in (or process if using a food processor) until the mixture resembles fine breadcrumbs.
Place the egg yolk in a cup and add the ice cold water and whisk together.
Add to the pastry mix and bring together to make a dough adding more water if the dough is too dry. If using a processor add the egg mixture and whizz until the mixture forms a ball of dough.
Wrap in clingfilm and place in the fridge for 30 minutes.
Meanwhile make the filling. Cook the potatoes in boiling water for 15 minutes, or until just tender.
Drain in a colander, tip back into the saucepan and cut into 2-3cm pieces.
Melt the butter and oil in the frying pan and sauté the onions gently for 15 minutes, or until soft and pale golden-brown.
Add the garlic and cook for a further two minutes, stirring regularly.
Preheat the oven to 200C/400F/gas 6.
Add the onions and garlic to the potatoes and sprinkle with half of the grated cheese and the parsley. Add the spinach leaves and season with nutmeg, salt and freshly ground black pepper. Mix all the ingredients together until well combined.
Remove the pastry from the fridge and place on a lightly floured work surface. Roll out to 3mm thick and line the tart tin with it.
Scrunch up a large piece of greaseproof paper, open it out and place over the pastry. Fill with baking beans and bake in the preheated oven for 15 minutes.
Take out of the oven, remove the paper and baking beans and brush the base with the beaten egg. Return it to the oven for a further 10 minutes or until the pastry is cooked and the egg wash is dry. Allow to cool a bit before adding the filling.
Spoon the filling mixture into the pastry case. Pour over the cream and allow it to drizzle down between the layers. Sprinkle the remaining cheese on top.
Place the pie on a baking sheet and bake in the preheated oven for 40-45 minutes, or until the pastry is crisp and pale golden-brown.
Remove from the oven and leave the pie to cool in the tin for 10 minutes.
Remove the pie from the tin and place it on a serving plate. Cut into thick wedges with a sharp knife.
