Step 1

Preheat oven to 350℉. Line a half sheet pan (13×18-inches) with parchment paper.⁣

Step 2

Rub together the sugar and lemon zest until the zest is evenly mixed through the sugar. Add the cornstarch. Stir to combine. Gently stir together the strawberries and rhubarb with the lemon juice. Then add the sugar mixture and stir until the fruit is evenly coated. Set aside.⁣

Step 3

Roll the dough out on a lightly floured surface into a 12-inch circle. Place the dough on the pan.⁣

Step 4

If using the jam and graham crumbs, spread the jam onto the pie dough, leaving a 2-inch border. Sprinkle the graham crumbs over the jam.⁣Gently spoon the filling onto the dough, leaving a 2-inch border of dough.⁣

Step 5

Fold the dough over the filling, pleating as needed. Brush the edge with the whisked egg and sprinkle sugar over the dough.⁣

Step 6

Bake until the filling bubbles in the center and the crust is golden brown, about 30 minutes.⁣

Step 7

Remove the pan from the oven and allow the galette to cool on the pan.⁣

Step 8

Store the leftover galette in the refrigerator for up to three days.