In the bowl of a food processor, combine the flour and salt.  Add the butter and pulse until the mixture resembles coarse crumbs. Combine the egg and water in a small bowl; add to the food processor and pulse until the mixture just begins to clump together. 

Transfer the mixture out onto a sheet of plastic wrap; fold the wrap over and press the crumbs until they hold together. Shape into a disc. Wrap and chill at least 30 minutes.

Turn the chilled dough out onto a lightly floured surface and roll it out to a 15-inch circle. Carefully roll the pastry around the rolling pin and transfer to a 9 x 2 1/2-inch springform pan, pressing it into the sides. Trim the overhanging pastry to 1 inch and press it firmly against the outside of the ring to help prevent it from shrinking. Use the trimmings to fill any cracks. Freeze the shell for 20 minutes.

Preheat the oven to 400°F.  Place the springform pan on a baking sheet. Line the pastry shell with a double thickness of aluminum foil long enough to overhang the sides. Bake about 20 minutes or until the edge of the dough is lightly browned. Remove the foil and continue baking the pastry shell for 10 to 15 minutes longer, or until lightly browned on the bottom. Transfer the baking sheet to a rack and let the pastry cool before filling.

In a large skillet over medium heat cook the pancetta until lightly browned, about 10 to 12 minutes, stirring occasionally. With a slotted spoon, transfer pancetta to a paper towel-lined plate, reserving 2 tablespoons drippings in the skillet.

Return thr skillet to medium heat. Add the onions and dried thyme; cook, stirring occasionally, until tender and golden, 10 to 15 minutes. Stir in the sun-dried tomatoes and cook 1 minute. Add the spinach and cook until wilted, 1 to 2 minutes. Remove from the heat and allow to cool for 10 minutes. Stir in the pancetta and cheese.

For the custard: In a blender combine the eggs, yogurt, milk, salt, pepper, and nutmeg. Cover and blend until frothy.

Set the springform pan with the pre-baked pastry shell on a baking sheet. Spoon the onion mixture into the bottom of the pastry shell. Pour the custard over the onion mixture. Bake for 1 hour 20 minutes to 1 hour 30 minutes or until the top is lightly browned and the custard is just set in the center (165°F). 

Let cool in the pan for 40 minutes. With an offset spatula or serrated knife, cut the pastry shell flush with the top of the pan. Carefully remove the springform pan ring. Garnish with fresh basil, if desired. Cut the quiche into wedges and serve while still warm.