Combine the 2 cups flour, 3 tbps. sugar, and pinch of sea salt together in a food processor. Pulse to combine and aerate.
Add in the 8 tbsp. cold butter cubes. Pulse to combine until the mixture reaches a sandy texture.
Add the 1/4 cup ice water in, tablespoon by tablespoon, until the dough is just combined and sticks together when pinched.
Gather all of the dough up into a ball, wrap in parchment paper, and refrigerate for 30 minutes.
Combine the rhubarb, blueberries, 1/4 cup sugar, and 1 tbsp. flour together in a bowl. Stir to combine and coat the fruit in the flour, gently. Set aside.
Remove the dough from the refrigerator and heavily dust a work surface. Use the palm of your hand to squish the dough into a rough circle shape before using a rolling pen to gently roll the dough ball into a 12″ round, dusting as necessary with flour to keep the dough from sticking.
Place the rough on a sheet of parchment paper.
Spoon the rhubarb and blueberry mixture into the center of the round, keeping a 3″ border along the outside of the dough free from fruit. Begin to use your fingers to gently fold the edge of the dough inwards towards the center of the circle, crimping and folding as necessary until all of the fruit is surrounded by the folded crust.
Place the parchment paper and the galette onto a baking sheet.
Whip the egg white in a small bowl with a fork before carefully brushing it onto the pastry edges of the galette with a pastry brush. Sprinkle the pastry edges with the remaining 1 tablespoons of sugar.
Bake in a preheated 375 degree oven for 50 minutes or until set and golden.
Allow the galette to cool before slicing and serving.