Step 1Heat the oven to 350 degrees. To make the pastry, sift the flour (if using unsalted butter, add a good pinch of salt to the flour and mix it in well). Add the butter cubes to the flour and rub the mixture with your hands until it reaches a crumbly texture, like breadcrumbs.
Step 2Stir in the large egg and 1 egg yolk. When the mixture is well combined, begin to knead. If the dough is too dry, add one scant tablespoon of cold water. Once the pastry forms a supple ball, knead a few minutes so it's well mixed. Cover the dough with plastic wrap and leave it to rest in a cool spot on the counter for at least 20 minutes for the glutens in the flour to relax. If you leave it to rest in the refrigerator for any length of time, be sure to remove it at least an hour before you plan to roll it out.
Step 3Lightly dust a clean working surface with flour. Roll out the pastry dough and fit in a 10 1/2-by-1-inch tart pan.
Step 4Place a layer of baking beads in the tart shell and bake for 15 to 20 minutes. Remove the tart shell from the oven, remove the beads and brush the tart shell with the beaten egg. Return the shell to the oven and bake for 15 minutes.
Step 5While the tart shell bakes, prepare the filling. Blanch the spinach, then drain it and press it between two plates to extract the water. Snip the cooked spinach with scissors into small pieces.
Step 6In a large mixing bowl, combine the spinach, the remaining 8 egg yolks, the cream and the Parmesan. Season with the nutmeg, salt and pepper.(If you're unsure about the seasoning, cook a dab of the mixture in a frying pan. But this should be perfect. Do not season more because too much nutmeg and pepper can create a soapy aftertaste.)
Step 7Remove the tart shell from the oven, quickly shutting the door to preserve heat. Pour the filling into the shell. Place the tart on a middle rack in the oven. Reduce the heat to 300 degrees and cook for 25 minutes.
Step 8Raise the heat to 350 degrees and cook for five more minutes. Be sure not to cook to firmness; the tart should have a slight jiggle when you remove it. Serve with an Orvietto or Vouvray and a salad.