Crust:

1. Mix together flour and salt in a medium bowl. Add the butter, then use a pastry cutter or a fork to cut the butter into the flour.  Do this until the butter is broken into pea-sized pieces, about two minutes.
2. Whisk together the egg and two tablespoons cold water, then pour into the flour mixture. Use a fork to stir until it just comes together, adding one or two additional tablespoons of water until it forms a ball.  Do not knead the dough or mix any further.  Press into a disk then wrap in plastic and refrigerate for at least 30 minutes, or up to 2 days in advance.

Filling & Assembly:

3. Melt butter in a medium skillet over medium-high heat. Add the leeks and cook, stirring often, until softened, about 6 minutes.  Season with salt and pepper, then add Dijon mustard, cream and 1 tsp rosemary.  Cook until the cream has reduced by half and the mixture is thick, about 5-8 minutes.  Taste for seasoning and adjust as needed.  Let cool completely.
4. When ready to assemble the galette, preheat the oven to 375°F and line a sheet pan with parchment paper. Roll the dough out on a floured surface until it is about ¼ inch thick and 14-16 inches around.  Gently roll the dough up onto the rolling pin and transfer it to the center of the sheet pan.  It should hang off the sides slightly.
5. Spread the cooled leek mixture evenly around the dough, leaving a 2-inch border on the outside. Crumble goat cheese on top, then arrange the pears in a circular pattern.  Sprinkle with remaining teaspoon of rosemary, salt, pepper, and chopped walnuts.
6. Carefully gather the outer border of dough and fold it up and over the filling to form a 1-to-2-inch crust. Brush the outer crust with egg wash if desired (this will produce a prettier, more golden-brown crust).
7. Bake for 30-40 minutes or until the crust is golden brown. Let cool for 30 minutes before slicing.  Serve warm or at room temperature.