In a medium bowl, mix together the flour and salt.

Add the butter and use a pastry cutter or fork to work it into the flour until the pieces are about the size of peas.

In a small bowl, whisk the egg with 2 tablespoons of cold water. Pour into the flour mixture and stir with a fork until the dough just begins to come together. Add 1 to 2 more tablespoons of cold water if needed until it forms a ball.

Do not knead or overmix. Shape the dough into a ball, wrap it in plastic, and refrigerate for at least 30 minutes before rolling out.

Melt the butter in a sauté pan over medium heat. Watch carefully as it sizzles, foams, and then turns golden brown with a nutty smell.

Add the minced sage, then immediately remove from the heat.

Place the pumpkin purée in a medium bowl. Pour in the browned butter, then add the egg and salt. Mix until fully combined.

Preheat the oven to 425°F. Line a baking sheet with parchment paper.

Roll out the chilled dough into a 12-inch circle and transfer it to the prepared baking sheet.

Spread the pumpkin mixture in the center, leaving a 2-inch border around the edges.

Evenly distribute the caramelized onions over the pumpkin, then crumble the goat cheese on top. Drizzle with honey and scatter the whole sage leaves evenly over the filling.

Fold the edges of the dough up and over the filling, pleating as needed.

Brush the exposed crust with egg wash and, if desired, sprinkle lightly with flaky sea salt.

Bake for 25 to 30 minutes, or until the crust is golden brown.

Let cool for at least 20 minutes before slicing and serving.