Add the flour, powdered sugar, and salt to a food processor and pulse a few times to combine.

Scatter the cold butter cubes over the dry ingredients and pulse just until the butter is broken up into pea-size pieces; a few larger chunks are fine.

Add the lemon juice and the chilled mascarpone, then pulse several times until the mixture looks like damp sand.

With the machine running, drizzle in 1 to 2 tablespoons of ice water and process just until the dough starts to clump and form a ball.

Turn the dough out onto a lightly floured work surface and gently knead it a few times to bring it together.

Pat the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes.

Chill until the dough is firm enough to roll, about 30 minutes total.

Position a rack in the center of the oven and preheat to 400°F. Line a large baking sheet with parchment paper.

On a lightly floured surface, roll the chilled dough into a rough circle about 1/4 inch thick; perfect edges are not necessary.

Transfer the rolled dough to the prepared baking sheet.

Spread the mascarpone cream over the dough, leaving a 1-inch border clear around the edges.

Arrange the peach slices over the cream in an overlapping pattern, keeping the same border.

Fold the exposed border of dough up and over the filling, pleating and pressing gently as you go to create a rustic crust.

Brush the dough border with the egg wash, then press the sliced almonds onto the egg-washed crust.

Bake for 25 to 30 minutes, until the center peaches are tender, the crust is golden brown, and the almonds are lightly toasted.

Cool the galette on the baking sheet for about 10 minutes.

Dust the almond crust generously with powdered sugar, slice, and serve warm or at room temperature.