Use a fork or pastry cutter to cut the butter into the flour, stopping when the mixture resembles course crumbs.
Mix in water one tablespoon at a time, adding just enough for dough to clump together in a rough ball (typically 3 to 4 tablespoons total).
Shape dough into a flat circle about 1-inch thick. Wrap in plastic wrap, then refrigerate for at least 1 hour.
Prepare Filling:
In a large bowl, combine strawberries, rhubarb, sugar, cornstarch, lemon zest, 1 tablespoon lemon juice, and a pinch of salt. Mix well. Set aside.
Assemble Galette:
Preheat oven to 400°F (200°C).
On a lightly floured surface, use a rolling pin to roll pastry into a 12-inch circle. Transfer to a parchment paper-lined baking sheet.
Spread filling over pastry, leaving a 1 ½ to 2-inch wide border empty.
Fold pastry edge over filling, overlapping where needed.
Whisk together egg yolk (save white for another use) and 1 tablespoon water. Brush egg wash over pastry.
Sprinkle coarse sugar over egg-washed pastry.
Bake galette until dough is deep gold brown and crisp (about 40 to 45 minutes).
Let galette rest for 20 to 30 minutes, then cut into wedges. Serve warm (or at room temperature) topped with ice cream or whipped cream, if desired. Enjoy!
Notes
Strawberry + Rhubarb Weights - For reference, I used a 16 ounce package of strawberries for this recipe, which ended up weighing 15.6 ounces / 440 grams. My sliced rhubarb weighed 8.4 ounces / 238 grams. You have some flexibility here, so don't worry if your weights don't equal mine exactly.Food Processor - If you have a food processor, you can use it to prepare the pastry crust (it will make the process faster and less messy). Just pulse in the butter into the salt / flour mixture, then add the ice water, stopping when the dough starts to come together in a rough ball.
More Tips - Read the information before this recipe card for helpful tips, step by step photos, recipe variations, and more!