In a mixing bowl, combine the flour and the salt. Quickly cut the butter into the flour, using a pastry blender or your fingers, until the butter pieces are the size of large peas. (Alternatively, cut the butter into the flour by pulsing it 8 to 10 times in a food processor, being careful not to overheat and overmix the butter.)
Dump the mixture out onto a clean surface and make a well in the center of the flour. Pour the ice water into the well. Using just your fingertips and working quickly, combine the flour mixture and the water. Work just until the water is absorbed. The dough will be ragged but should hold together when you squeeze it. If it seems dry, sprinkle on a few more drops of water. Form the dough into a log shape about 8 inches long and parallel to the edge of your work surface. With the heel of your hand, push down and away from you all along the line of dough. With a pastry scraper, gather up the dough, shape it back into a log, and repeat the smearing action. This technique, known as fraisage, will form sheets of butter in the dough, creating a light crust almost like puff pastry, with the pastry scraper, gather the dough up into a ball; it's fine if the dough does
not come together completely at this time.
Wrap the dough well in plastic wrap, flatten it a bit, and let it rest in the refrigerator for at least a half hour before rolling. The dough will keep in the refrigerator for up to 2 days. You can also freeze the dough, well wrapped; allow it to defrost for a day in the refrigerator before using it.
On a lightly floured surface, roll out the dough so it is slightly larger around than the 10-inch tart mold and about 1/8 inch thick. If your rolling yielded something unlike a circle, use a knife to trim the raggedy edges, but keep the size of the circle larger than the tart pan. Carefully lift the dough over the tart pan and allow it to fall into the pan, centering it fairly well. Gently ease the dough into where the sides of the pan meet the bottom. If necessary, lift the dough that's hanging over the edge and bring some of the excess down into the pan to ensure that the dough is following the pan's contours. Once the dough covers the entire bottom of the pan, fold the excess dough over into the pan to make thicker walls. Press the dough into the edges of the pan and build up the wall of dough slightly so that it's a bit higher than the edge of the pan. Refrigerate the dough for at least 30 minutes before baking.
To blind-bake the tart crust heat the oven to 375°F (190°C). Line the mold with aluminum foil, and then fill the foil with baking weights, dried beans, or rice. Bake for 12 minutes. Remove the foil and beans and continue to bake until the crust is well browned. Remove from the oven and let the crust cool a bit before assembling your tart.
MAKE THE FILLING
Heat the oven to 350°F (175°C). Wash the beets and dry them with a paper towel. Place the beets in a small ovenproof pan, drizzle them with the olive oil, and season with a little salt and pepper. Cover the pan with aluminum foil. Bake until the beets are tender when pierced with a paring knife, about 1 hour. Allow the beets to cool. Peel the beets using a small knife and cut them into a medium dice. (Be careful, as beet juice can stain counters, towels, and even your hands; you may want to wear gloves for this step.)
Heat the butter in a sauté pan over medium heat. Add the onion, season with a little salt, and cook, stirring every few minutes, until the onion is just tender, about 7 minutes. Add the white wine and cook for another minute, scraping up any browned bits stuck to the bottom of the pan.
Heat the oven to 350°F (175°C). Toss the beets and onion together and put them into the blindbaked tart shell. Whisk together the eggs and cream, season well with salt and pepper, and carefully pour over the beets and onion, letting the cream seep evenly into the beets. Dot the goat cheese all over the top of the tart. Put the tart on a baking sheet and bake it for 20 minutes.
Sprinkle the chopped walnuts on top of the tart and drizzle the walnut oil over it, if using. Return the tart to the oven and bake until just set, an additional 15 to 20 minutes. Sprinkle the tart with the chopped parsley and let it rest for at least 5 minutes before serving.