Add the flour, 1 tsp salt and 1 tsp sugar to a food processor. Whirl to blend
Add the 1 cup of cold butter cut into small pieces to the food processor. Process for 10 seconds until the mixture resembles course meal.
While the processor is running add the cold water 1 TBS at a time. Add just enough water to hold the dough together.
Remove the pie dough from the processor and shape into a disc on a floured surface. Wrap tightly in plastic wrap and refrigerate for at least 30 minutes or up to a few days.
Preheat oven to 375 degrees. Line a baking sheet with a piece of parchment paper
On a lightly floured surface roll out the dough into a 14 inch round. Place on the baking sheet. Set aside.
Place the rhubarb and juice of 1/2 lemon in a bowl and toss to coat.
Add 3/4 cup sugar, 1/4 cup flour, 1/4 tsp salt, 1/4 tsp cinnamon, 1/8 tsp nutmeg to the rhubarb and mix to thoroughly coat.
Arrange the rhubarb on top of the dough in the center leaving about a 2-inch border of dough all the way around it.
Fold the dough border up and over the rhubarb, overlapping it and pressing the sides against one another leaving the center open and the rhubarb showing. See image.
Brush the dough with the egg wash, sprinkle it with sugar and dot the rhubarb with the remaining 1 TBS unsalted butter.
Put the galette in the refrigerator for 20 minutes.
Bake until the crust is golden brown an the rhubarb is bubbly, about 55- 60 minutes.
Move to a wire rack to cool.
The rhubarb galette can be served warm or cold. Best with ice cream!