Methods

1. To make the pie dough, combine the flour, sugar, salt, and nutmeg in a medium bowl, and mix well. Add the butter and, using your fingers, rub the butter cubes into the flour mixture until they are the size of chickpeas. Add the water and gently mix until the mixture holds together when squeezed. Press the dough into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour or up to 2 days ahead (If you’re rolling the dough a day later, it might be too cold and crack when rolling. Let it sit at room temperature for 5 minutes for easier rolling.)
2. While the pie dough is chilling, make the filling. Combine the butter, sugar, and maple syrup in a large, ovenproof skillet and place over medium heat. Cook, stirring frequently, until the butter is melted and the sugar is dissolved, then continue to cook until the mixture starts to turn from very light golden to golden, about 2 to 4 minutes.
3. Stir in the pitted cherries, mace, and kosher salt. The butter-sugar mixture might seize a little bit, but it will dissolve again once the cherries release their juices. Continue to cook, stirring frequently, until the sauce is glossy and fully liquid again and the cherries just start to soften. Turn off the heat and let cool for 10 minutes.
4. In a small bowl, combine the cornstarch, lemon juice, vanilla extract and 1 tablespoon water, and mix until smooth. Add the cornstarch slurry, raspberries, and blackberries into the cherry mixture and gently fold everything together until combined, being careful not to break up the berries too much. Set aside.
5. Preheat the oven to 400°F.
6. To prepare the dough, unwrap the refrigerated dough and place on a lightly floured surface. Roll the dough out to about ¼” thick oval (don’t worry about getting the size perfect, you’re going to be cutting it up anyway). Cut the oval into 2½ x 3” rectangles and then cut each in half diagonally to create triangles. Place the triangle pieces of dough on a plate or sheet pan and freeze until chilled and firm, about 5 to 10 minutes.
7. Artfully arrange the dough triangles, overlapping some, on top of the filling in the skillet. Brush the top of the dough with the egg wash. Mix the demerara sugar and cinnamon in a small bowl and then sprinkle a little bit over the top of each piece of dough. Place the skillet on a baking sheet (just in case any of the filling bubbles over) and bake for 20 minutes. Reduce the heat to 350°F and continue to bake until the pie dough is cooked through and golden, about another 20 to 25 minutes. Remove from the oven and let cool for at least 15 minutes before serving.
8. Serve warm or at room temperature with lightly sweetened whipped cream or vanilla ice cream.