To make the crust:
Combine the flour, hazelnut flour, powdered sugar and salt in a food processor. Add the cubed butter and toss to coat. Pulse in short bursts until the butter is the size of small peas. With the food processor running, add the water a tablespoon at a time until the mixture comes together into a ball.
Remove the dough from the food processor and shape into a disk. Cover with plastic wrap and chill for at least 2 hours.
To assemble the galette:
Preheat to the oven to 375°F and line a rimmed sheet pan with parchment paper.
Roll the crust out on a lightly floured surface until it is a rough 12-inch circle. In a large bowl, toss together the cherries, cornstarch, sugar, lemon zest and lemon juice. Dump into the center of the circle, leaving a few inches around the edges.
Fold the edges up over the cherry mixture, pleating the dough as you go, until you've created a rim around the whole galette. Brush with egg wash and sprinkle with coarse sugar.
Bake until crust is golden and filling is bubbling, about 35-45 minutes. Let cool completely. Serve at room temperature or reheat to serve warm.
To make the whipped cream:
In the bowl of a stand mixer fitted with a whisk attachment, beat the heavy cream until thickened. Add the sugar and vanilla and beat until soft peaks.
If desired, serve galette with whipped cream and chopped hazelnuts.
Calories: 671kcal, Carbohydrates: 72g, Protein: 8g, Fat: 42g, Saturated Fat: 20g, Polyunsaturated Fat: 2g, Monounsaturated Fat: 10g, Trans Fat: 1g, Cholesterol: 85mg, Sodium: 208mg, Potassium: 436mg, Fiber: 6g, Sugar: 47g, Vitamin A: 1154IU, Vitamin C: 12mg, Calcium: 79mg, Iron: 3mg