Actress Alexa Vega joins Martha to make a rhubarb galette.
Preheat oven to 375 degrees. Line a baking sheet with parchment paper; set aside.
On a lightly floured work surface, roll out dough to a 14-inch round, about 1/8-inch thick.
Place rhubarb and lemon juice in a large bowl; toss to combine. In a medium bowl, whisk together flour, sugar, salt, cinnamon, and nutmeg until well combined. Add sugar mixture to rhubarb and toss until well coated. Arrange rhubarb mixture on top of dough, leaving a 2-inch border all the way around. Fold border over the fruit mixture, overlapping where necessary and pressing gently to adhere the folds. Brush edges of dough with egg, sprinkle with turbinado or sanding sugar, and dot with butter. Transfer to refrigerator and chill 15 to 20 minutes.
Bake until crust is deep golden brown and juices are bubbling, 55 to 60 minutes. Transfer baking sheet to a wire rack to cool the galette. Serve warm or at room temperature.