In a food processor, pulse the flour, almonds, rosemary and kosher salt until the almonds are finely ground. Scatter the cubed butter over the dry ingredients and pulse until pea size pieces of butter form.
In a small bowl, whisk the cold water with the egg yolk (reserve the whole egg for brushing). Add the yolk mixture 1 tablespoon at a time, pulsing between additions, until you can pinch the galette dough between your fingers. If needed, add more cold water 1tablespoon at a time.
Layer two large sheets of plastic wrap on a work surface and dump out the galette dough. Press the dough into a compact disc and wrap tightly in plastic wrap. Refrigerate for at least 2 hours.
Preheat the oven to 375°F and line a large rimmed baking sheet with parchment paper. Remove the galette dough from the refrigerator and let it soften slightly. Lightly dust a work surface with flour and then roll out the dough into a rough 12-inch circle, about ⅛-inch thick. Transfer the crust to the prepared baking sheet.
Arrange the sliced brie in the center of the galette, leaving a 2-inch border. Dollop thejam ontop of the brie. Fold the edges of the dough up over the filling, overlapping in softfolds. Brush the dough with the egg wash and sprinkle with flaky sea salt.
Bake the galette at 375°F for 30 to 40 minutes, or until the crust is golden brown and thefilling is bubbling. Transfer the baking sheet to a rack and let cool for at least 10 minutes.