To make the pastry, put the flour, butter and salt in a food processor and blitz until it looks like breadcrumbs. Add the egg yolk and pulse, adding just enough cold water so that the pastry starts to come together in a ball. Roll the pastry into a smooth ball, wrap in cling film and chill in the fridge for at least an hour.
Meanwhile, melt a knob of butter with a splash of oil in a small frying pan. Add the onion and fry gently for 10 minutes, until soft and translucent. Remove from the heat and leave to cool. Boil or steam the spinach leaves for 2–3 minutes, until wilted. Drain and refresh in a bowl of ice-cold water.
Once cooled, drain again and squeeze the spinach really well several times to get rid of any excess liquid. Roughly chop the leaves. Heat the oven to 180°C/Gas 4. Lightly flour a work surface and rolling pin. Lightly grease a 23cm tart tin with butter. Roll out the pastry to a rough circle, a little bigger than needed to line the tin.
Lay the pastry in the tin, pressing it into all the crevices. Trim any overhanging pastry. Chill the pastry in the fridge for a further 5 minutes. Put a baking sheet in the oven. Line the pastry case with baking parchment and fill with baking beans or dried, uncooked pulses or rice, making sure all the crevices are filled so the pastry doesn’t rise.
Place the tin on the hot baking sheet and bake for 15 minutes. Lift out the paper and beans, and bake for a further 10 minutes. Remove from the oven, brush lightly with just enough egg white to coat and seal the base (this helps prevent a soggy pastry bottom), then leave to cool completely.
Once the pastry has cooled, beat the eggs for the filling in a large bowl. Stir in the crème fraîche and crumble in the feta. Add the spinach, onion, dill and half the olives. Season with salt and pepper. Gently combine then pour the mixture into the pastry case. Press the rest of the olives into the top so they’re visible once baked. Bake for about 30 minutes, until the filling has just set. Remove and leave to cool to just warm or room temperature before cutting into slices to serve.
* Use chard leaves or spring or summer greens instead of spinach. To include chard stems, chop them small and fry along with the onion. In winter, substitute with shredded kale or even cabbage.
* Try other veg, such as fried courgette, asparagus or beans.
* Add a few sun-dried tomatoes with the olives.
* Vary the herbs – try thyme, basil, oregano or marjoram.
* Add some garlic – fry some shredded wet garlic with the onion, or add some shredded wild garlic leaves just before baking.
* Use chopped spring onions instead of onions (these need minimal frying).
* Ring the changes with the cheese – try blue cheese, Cheddar,
Parmesan, or simply use up what you have in your fridge.