1 cup Unsalted Butter (Chilled, cut into small pieces)
1/4 cup Ice Water
1-1/2lb Nectarines (Halved, Pitted, and Sliced Lengthwise)
1/2 pint Raspberries
3 tablespoons Granulated Sugar
1 Large Egg Yolk (Whisked with 1 t. water, for egg wash)
1 tablespoon Coarse Sugar
To make the galette dough, put the flour and salt in a food processor and pulse a few times to blend. Add half the butter and pulse just until the fat is evenly distributed and coated with flour. Add the remaining butter and pulse a few times, just until it is coated with flour. There should still be pieces of flour-coated butter about the size of large peas. Transfer the mixture to a bowl. Drizzle in the ice water while tossing with a fork, adding water just until the mixture begins to come together. Gather the dough with your hands and knead gently if necessary to get it to hold together. Resist the temptation to add more water. Handling the dough as little as possible, shape it into a thick disk like a hamburger patty, then wrap it in plastic wrap and refrigerate until chilled, at least 2 hours or up to 1 day.
Preheat the oven to 425°F. If you have a baking stone, put it in the oven on the middle rack to preheat for at least 45 minutes.
Remove the dough from the refrigerator about 10 minutes before you are ready to roll it to allow it to soften slightly. If it is too cold, it will be brittle and difficult to roll. Unwrap the dough and place it between 2 sheets of parchment paper at least 16 inches square. With a rolling pin, flatten the dough into a 15-inch circle. To prevent sticking, occasionally lift the top sheet of parchment and lightly flour the dough, then replace the parchment and invert the dough so the bottom sheet is on top. Peel back that sheet of parchment and lightly flour the dough, then replace the parchment and continue rolling.
When the circle is large enough, remove the top sheet of parchment and transfer the dough, still on the bottom sheet of parchment, to a rimless baking sheet. Working 2 inches from the edge of the dough, place the nectarine slices in concentric rings. Scatter the raspberries evenly over the nectarines. Sprinkle the fruit with the granulated sugar.
Using a palette knife or other broad knife to help you lift the dough off the parchment, gently fold the uncovered edge of the dough over the fruit to make a wide, roughedged rim. Make sure there are no cracks in the dough for juices to seep through. Brush the rim with the egg wash, then sprinkle the rim with the sparkling sugar. With scissors, cut off the exposed parchment paper. (It's okay if a little paper is exposed.)
If you have a baking stone in the oven, slide the galette, still on the bottom sheet of parchment, directly onto the stone. If you do not have a baking stone, bake the galette on the rimless baking sheet. Bake until the crust is golden and the nectarines are juicy, about 50 minutes. Slide the galette onto a rack to cool slightly. To serve, slide it from the rack onto a cutting board and cut into wedges. Serve warm.