Soften the leeks in the butter for a couple of mins. Add the spinach to the pan, cover and cook for 5-6 mins more until the spinach has just thawed.
In a separate bowl, beat the ricotta and eggs with the parmesan, spinach mix, nutmeg and plenty of seasoning. Stir in the artichokes and tomatoes.
Heat oven to 180C/fan 160C/gas 4. Line the base and sides of a greased 23cm clip-sided or loose-based tin with the filo pastry. Brushing each sheet lightly with the oil, place in the tin, oil-side down, leaving excess to hang over the edge. Turn the tin a quarter turn after each sheet.
Tip filling into the tin, fold excess pastry onto the top of the pie, a sheet at a time, crumpling to give a ruffled effect. Bake for 11⁄2hrs until golden and firm.