Creating a Rhubarb & Blueberry Galette can be straightforward if you follow these simple steps:

1. Prepare the Crust: In a bowl, combine flour, sugar, and salt. Add the cubed butter and mix until the mixture resembles coarse crumbs.
2. Add Ice Water: Gradually add ice water, mixing until the dough comes together. Form it into a disk, wrap in plastic, and refrigerate for at least 30 minutes.
3. Preheat Oven: Preheat your oven to 375°F (190°C) while the dough chills.
4. Prepare the Filling: In a bowl, mix diced rhubarb, blueberries, sugar, cornstarch, lemon juice, and vanilla extract. Set aside to let the flavors meld.
5. Roll Out the Dough: On a lightly floured surface, roll out the chilled dough into a circle about 12 inches in diameter. Transfer it to a parchment-lined baking sheet.
6. Add the Filling: Spoon the fruit mixture into the center of the dough, leaving a 2-inch border around the edges.
7. Fold the Edges: Gently fold the edges of the dough over the filling, pleating it as you go to create a rustic look.
8. Egg Wash: Beat the egg and brush it over the exposed dough edges. Sprinkle coarse sugar on top for added texture and sweetness.
9. Bake: Place the galette in the oven and bake for 40-45 minutes or until the crust is golden brown and the filling is bubbling.
10. Cool: Once done, remove the galette from the oven and let it cool for about 10-15 minutes before serving.

These steps will guide you in creating this incredible dessert effortlessly.