To make the dough, plus all the ingredients while drizzling in the ice water, in a food processor until it comes together. Place in bowl and let rest for 15-30 minutes.
In the meantime, preheat the oven to 375F and sprinkle a baking tray with cornmeal. In the same food processor, grind the pistachios for the filling and then blend with ricotta and extract to form a nutty paste.
Roll out the dough on a floored surface and spread the ricotta/nut paste at the bottom, in a circle about 5 inches wide. Layer the sliced plums around, tuck the galette over, and sprinkle a few more pistachios on top. Slide on to the baking tray, brush an egg wash over the dough, and bake until golden brown round the edges, about 30 minutes.
Serve warm or room temperature with a dollop of whipped cream and crushed pistachios.