Combine 1-1/4 cups flour, 2 teaspoons sugar and salt in a large bowl. Use a fork or pastry blender to cut the butter into the flour until the mixture resembles pea-sized clumps. Use a spatula or wooden spoon to gradually fold the water into the dry ingredients. When the dough starts to form, use your hands to bring it together into a ball. Press the dough into a disk and cover with plastic wrap. Chill in the refrigerator for at least 1 hour or up to 2-3 days.

Preheat the oven to 375 degrees F.

In a large bowl, gently stir together the plums, 1/4 cup sugar, 2 tablespoons flour, lemon juice and lemon zest.

On a lightly floured work surface, roll out the dough into an 11-inch diameter circle. Transfer to a parchment paper-lined sheet pan. Start in the center and arrange the plum slices overlapping in a circular pattern, leaving a 2-inch border. Fold the dough at the edge over to meet the fruit.

Whisk together the egg with 1 tablespoon water. Brush the egg wash on the crust and sprinkle with sanding sugar. Bake for 45 minutes, until golden brown. Let cool and serve warm or at room temperature.