In a medium mixing bowl, whisk together the all-purpose flour, sage, and salt. Use clean hands to work the butter into the dry ingredients until they are the size of small peas. Drizzle in the ice water and stir until the dough is evenly moistened. Use your hands to press the dough into a ball. Gently press into a disk, wrap in plastic wrap, and let chill in the fridge for at least 30 minutes.

Meanwhile, bring a large saucepan of salted water to a boil over high heat. Add in the potato slices and cook until a fork easily pierces the slices, about 5 to 10 minutes depending on the size of the potato slices. Drain and set aside.

In a medium mixing bowl, whisk together the ricotta, 1/4 tsp salt, dash of pepper, and 1 large egg until combined.

Preheat oven to 425 degrees and lightly grease a baking sheet.

On a lightly floured clean surface, roll out the dough into a 12" circle. Transfer the dough to the prepared baking sheet. Spread the ricotta mixture over the dough, leaving a 2" outer ring bare.

Top the ricotta with slices of potato and apples, overlapping as needed to make them all fit.

Fold the bare outer parts over the filling to create a crust. Whisk the remaining egg with 1 Tbsp water in a small bowl and brush over the edges exposed dough.

Bake for 25 to 30 minutes or until the crust is golden brown and the filling is starting to brown.

Meanwhile, transfer the apple cider to a medium saucepan and bring to a simmer over medium-high heat. Let simmer, stirring often, until the cider has reduced by half, about 10 minutes. Add the mustard and continue to simmer for another 2 minutes. Remove from heat and stir in the butter which should help thicken up the glaze a bit.

Remove galette from the oven and liberally brush the glaze all over the galette. Let cool for 10 minutes before cutting. Garnish with sage, sprinkle with flaky salt, slice and serve the remaining glaze on the side.