Place your flour, sugar, and salt in a large bowl and whisk to combine.

Cut your butter into ½ inch cubes and add them into the flour mixture.

Toss them around so that they’re coated and use your fingers to roughly smash each butter cube into flatter pieces.

Pour the cold water over butter-flour mixture and knead until the dough forms a ball.

Divide dough into two parts, and turn onto a well-floured work surface. Roll each section into a rectangle and fold the rectangle into thirds, then then in half, and wrap in plastic wrap.

Chill in the fridge until firm — one to two hours.

In a large bowl, combine the strawberries, rhubarb, sugar, lemon juice, and tapioca. Toss to combine and set aside until the crust is ready.

Preheat oven to 425F.

Roll out one half of the dough to an 11x15 rectangle and transfer it to your quarter sheet pan.

Spoon in the filling and spread to the edges.

For the top crust, either roll out the second sheet and seal the edges, or cut into strips for a lattice top.

Brush crust with the beaten egg and sprinkle with sugar.

Bake for about 45 minutes, or until the filling seems bubbly and juicy.

Allow to cool before slicing.