- In a large bowl, whisk together flour, sweetener, and salt. Use a box grater to grate butter over flour. Gently rub butter into flour to incorporate. Set aside.
- In a measuring cup with a pour spout, stir together water, vinegar, and ice. Pour half the water over the dry mixture and gently toss to combine. Incorporate more of the water and gently knead until dough just comes together into a shaggy mass. Flatten into a disc, wrap in plastic wrap, and chill at least 1 hour or overnight.
- Line a large rimmed sheet pan with parchment paper. Lightly flour a countertop and a rolling pin. Roll out the dough, applying even pressure with the rolling pin, into a 14” circle. Transfer dough to prepared sheet pan and refrigerate while making filling.
- In a large bowl combine pluot halves and blueberries and toss to coat with sweetener, lemon zest and juice, and flour.
- Arrange fruit mixture on crust, leaving a 2” border around the edge. Fold edges of dough up and over the fruit, pressing firmly to seal the folds. Chill until the rim of the dough is firm, 15-30 minutes.
- Preheat oven to 400° F. Remove galette from the refrigerator, brush crust edges with egg, and sprinkle with sugar.
- Bake until deep golden brown and juices are bubbling throughout, 30-40 minutes. Let cool slightly, 10-15 minutes. Serve warm or at room temperature.