Pulse together the flour, sugar and 1/2 teaspoon salt in a food processor. Add about a 1/3 of the chilled butter cubes and process until the butter is absorbed. Add the remaining butter and pulse until the butter is the size of peas, 5 or 6 pulses. Add the vinegar. Then, pulsing quickly a few times as you do it, add the ice water through the tube. Squeeze some dough between your fingers; it should just hold its shape without feeling wet. If it is crumbly, add more ice water a tablespoon at a time. Pulse quickly; do not overwork or the dough will be tough. Wrap the dough loosely in a large piece of plastic wrap, then firmly press and flatten the dough into a thin round. Refrigerate for 1 hour.
Lightly dust a work surface with flour, roll the dough out into an 11- to 12-inch round, about 1/8 inch thick. Center the dough in a 9-inch pie plate, with an even overhang all around. Fold the edges and flute or crimp. Chill for at least 30 minutes.
Preheat the oven to 375 degrees F.
Press a piece of foil over the chilled crust, fill with pie weights, raw beans or rice. Bake to set the crust, about 20 minutes. Remove the foil and weights and bake until the crust is light brown, 5 to 10 minutes more. Let cool slightly before adding the filling.
Adjust the oven temperature to 350 degrees F.
For the filling:
Sprinkle the onion evenly into the crust. Combine the cheese, ham and spinach in a small bowl and sprinkle it in an even layer over the onion.
For the custard:
Whisk together the half and half, parsley, eggs, 1/2 teaspoon salt and 1/4 teaspoon pepper in a small bowl.
Pour the custard over the filling and bake until the quiche is set, 35 to 50 minutes. Let cool at least 30 minutes before serving warm or at room temperature.
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