THE PASTRY: In a medium bowl, blend the flour and salt. Using a small knife, cut the butter into pieces and drop into the flour mixture. Using your fingertips, gently work the flour into the butter, until pea-sized clumps form. (If your kitchen is warm, place the mixture in the refrigerator for a minute or two to prevent the butter from melting.)
One tablespoon at a time, drizzle the water evenly over the flour mixture and stir gently to form a dough (depending upon the temperature and humidity, you may need more or less water). Gather the dough into a ball; it should hold together. If not, add a little more water. Being careful not to overwork the dough, pat it into a disk, wrap it in plastic wrap or parchment paper, and let rest for 30 minutes in the refrigerator.
Heat oven to 350°F. On a floured
surface, using a flour-dusted rolling pin, roll the dough to an 11-inch circle. Gently press into the bottom and sides of a 9-inch tart pan; trim away any excess.
THE FILLING: In a small bowl, whisk the egg yolk into the cream. In the pastry shell, arrange the potato slices in overlapping layers. Top with the cheese. Pour the cream mixture over the potatoes and cheese. Sprinkle the tart with chopped rosemary or thyme.
Bake the tart on the bottom rack of the oven for 40 to 50 minutes, or until the top is golden brown and filling is set. Remove from the oven, sprinkle with fleur de sel, and transfer to a baking rack to cool. If using a removable-bottom pan, gently push up from the bottom while the tart is still warm to help release the crust.