Make the crust: Pulse the flour, sugar and salt in a food processor. Add half of the butter and pulse until the mixture looks like coarse meal. Add the remaining butter and pulse into pea-size pieces. Add 1/4 cup ice water and pulse until the dough clumps together but has not yet formed a ball. Turn out the dough onto a piece of plastic wrap; press into a disk and wrap. Refrigerate until firm, at least 1 hour or overnight.
Meanwhile, make the filling: Pulse 1/2 cup almonds, 1/4 cup sugar, the egg yolk, butter, vanilla and lemon zest in a food processor to make a paste.
Preheat the oven to 400 degrees F and line a baking sheet with parchment paper. Roll out the dough into a 12-inch round on a lightly floured surface. Transfer the dough to the prepared baking sheet. Spread the almond filling over the dough, leaving a 2 1/2-inch border; top with the grapes. Fold the edge of the dough over the fruit by about 2 inches, pleating as needed. Refrigerate until firm, 30 minutes.
Lightly beat the egg yolk with 1 tablespoon water. Brush the dough with the egg wash. Scatter the remaining 2 tablespoons almonds all over the galette, then sprinkle with the remaining 2 tablespoons sugar. Bake until the crust is golden brown and the grapes are softened, about 40 minutes. Let cool on the baking sheet.