In a large jug, mix the yeast with 175ml warm water. Leave for at least five minutes for the yeast to froth a little. Add the yeast and water to the flour and mix vigorously with a metal spoon until the dough comes together. Alternatively, use a food processor fitted with a dough hook. Mix or knead well to form a smooth, cohesive dough – this should take at least 5-6 minutes.
Remove the dough from the mixer, if using, and fold the dough in on itself a few times using lightly oiled hands. Wipe out the bowl. The dough should be smooth and elastic, with a good sheen to it. Put the dough back in the bowl and cover with a clean, damp tea towel. Prove in a warm place until doubled in size, about two hours.
Meanwhile, sweat the leeks in a pan with the butter over a moderate heat to soften, about eight minutes. Do not let them colour. Remove from the heat and leave to cool.
Preheat the oven to 220C/200C fan/gas mark 7 and line a 40 x 30cm rimmed baking tray with greaseproof paper.
Beat the eggs in a bowl. Add the crème fraîche or sour cream, thyme leaves, nutmeg, and salt and pepper to taste. Add the cooled leeks.
Lightly flour a work surface and roll the dough out to fit the tray. It should be about 5mm thick.Make a small lip around the edge of the dough, or push it right out and up to the very edge of the tray.
Add the leek mixture, distributing it evenly. Lay the potato slices on top and finally scatter over the Parmesan.
Bake for 20-25 minutes, until the top is nicely coloured and the base is crisp. Remove from the oven, cut into squares and serve.