Preheat oven to 200°C. Grease a 26-cm springform tin.
Mix sour cream, 1 egg, a pinch of salt and 3 tablespoons oil in a bowl. Sift the flour and baking powder over cream mixture; knead into a dough. Roll into a ball, wrap with cling film and refrigerate while preparing the filling.
Heat 2 tablespoons oil and the butter in a wide saucepan. Add leeks and sauté until transparent. Sprinkle in 1 tablespoon flour, add milk and stir to combine.
Bring mixture to the boil.
Remove pan from heat; leave to cool. Stir in ricotta, 3 eggs, extra 2 tbsp sour cream, the parsley and two-thirds of the pine nuts. Season with salt and pepper.
Roll out the pastry thinly on a lightly floured work surface and line the tin. Pour the leek mixture into pastry case and sprinkle with the remaining pine nuts. Sprinkle with nutmeg.
Bake tart for 40 minutes until lightly browned. Serve warm.