Make the Shortcrust Pastry, recipe follows, leaving to chill for 30 minutes before using. Preheat the oven to 350 degrees F. Line the tart tin with the dough and bake 'blind'. Cook's Note: Blind Baking. To pre-bake a pastry shell before adding filling, first chill the pastry, and then line your pan. Line the pastry with foil, parchment or greaseproof paper, leaving plenty to come up the sides. Fill with baking beans (alternatively, for a really smooth finish use plastic wrap to line the pastry: place 2 sheets over the pastry, add the beans and bring the edges into the center.) Whichever method you choose, bake 'blind' in a 350 degree F oven for 15 to 20 minutes, or until the pastry feels dry. Remove the paper and beans, and brush with a little leftover beaten egg, and return to the oven for 2 minutes. If there are any little holes or cracks in the pastry, just patch it up with any leftover raw pastry before you return it to the oven. Remove from the oven, and set aside in the pan while you make the filling. The pastry can be baked a day in advance and kept covered until you need it. Filling: Wash and dry the spinach, removing any tough stalks and stems, if using large spinach leaves. In a medium-size saucepan, cook the spinach in just the water that's clinging to it, over a low heat until it wilts. Drain the spinach in a colander or sieve, and let cool a little, then squeeze most of the moisture out with your hands, and roughly chop. Meanwhile, steam or boil the potatoes, until just cooked, and cool the potatoes on a tray or board. When the potatoes are cool enough to handle, cut them into 1/4-inch thick slices. In a medium-sized bowl, whisk the eggs, and add the cream, salt, and pepper, lemon zest, nutmeg, cayenne pepper, and grated Parmesan. Whisk these ingredients together, and then add the spinach, and mix through. Season well as the potatoes are very mild and need a good contrast. Spread the potato slices over the base of the prepared tart, and dot with the goat's cheese, reserving a few slices of each to place on the top, if you wish. Gently spoon the spinach cream mixture over the cheese, as high as you can go. If you are concerned about spillage, carry the tart minus the last few spoons of filling over to the oven. Place the pan in the oven, and then add the remaining filling, and any remaining potato, and cheese slices. Bake until the tart is golden brown and just set in the center, about 35 to 40 minutes. Remove the tart from the oven, and let cool for about 10 minutes before serving.
Place the flour, salt, and butter in a food processor and whiz or pulse briefly. Add half the beaten egg and continue to whiz. You might add a little more egg, but not too much as the mixture should be just moist enough to come together. If making by hand, rub the butter into the flour until it resembles coarse bread crumbs. Then, using your hands, add just enough egg to bring the dough together. With your hands, flatten out the ball of dough, until it is about 3/4-inch thick, and then wrap in plastic wrap or place in a plastic bag and leave in the refrigerator for at least 30 minutes or, if you are pushed for time, in the freezer for 10 to 15 minutes. Makes 1 crust (about 14 ounces).