Heat the oven to 180C
Grease the side of the pie dishes (11cm wide x 5cm deep) with butter
Cut out 4 x 20cm circles from the two sheets of pastry
Gently push the pastry into the pie dishes, making sure it goes into the corners and sticks to the sides
Prick holes in the bottom of the pastry using the prongs of a fork
Cut out 4 x 20cm circles from baking paper and push each one into the pie dish and tuck it over the edge so the pastry is covered
Fill each one with around 2-3cm baking beans
Bake in the oven for 10 minutes
Lift the paper and beans from the pie cases and bake for another 5 minutes
Whilst the pie crust is baking, heat a drizzle of oil in a large non-stick pan
Add the onion and fry for 5-7 minutes until soft
Place the mashed potato into a mixing bowl and gently warm through in the microwave so that it's easier to stir
Mash it with a fork
Add 100g grated cheese, chopped fresh herbs, fried onions, chopped spinach and milk to the bowl
Season with salt and pepper and give it a good stir until all the ingredients are combined
Fill the pie cases with the mash until filled to the top
Cover in grated cheese and return to the oven for 20 minutes until golden brown