Here's how: 

1 Preheat oven to 200°C. Put the oil, butter, onion and garlic in a pan over a medium heat and cook for 3 minutes. Add the leek, cook for 5 minutes, then add water, lower heat and cook for a further 3 minutes. Add the spinach and cream and cook for 2 minutes. Remove from heat and cool. Puree cooled leek paste with a stick blender until smooth.

2 Chill a 4.5cm-deep 23cm fluted loose-bottomed tin in the fridge. Spray tin with oil. Lay the pastry in the tin and push to the bottom carefully without piercing the pastry. Spread the cooled leek paste over the bottom of the tin and 3cm up the edges. Crack the eggs over the paste. Top with the pancetta and chives and season with salt and pepper.

3 Roll the edges of the pastry in a rope-like fashion down to the green-paste level. Season the eggs generously with salt and pepper. Put the tin on a flat, heavy cast-iron tray. This is important to keep enough heat under the pie to cook the pastry from the bottom. Bake for 25-30 minutes.