In a large bowl, whisk together all-purpose flour, fine sea salt, and granulated sugar until airy and well-combined. Set this mixture aside as you prepare the next step.

Add cold, cubed unsalted butter to your dry mixture, using a pastry cutter or your fingertips to blend until the mixture resembles coarse crumbs.

Incorporate the sourdough discard and white vinegar into the butter-flour mixture. Gently stir until a shaggy dough begins to form.

Wrap the shaggy dough tightly in plastic wrap and refrigerate for at least 30 minutes.

Once chilled, lightly flour your work surface and roll the dough to about ⅛-inch thickness.

Using a sharp knife or a pizza cutter, slice the rolled dough into 3x4 inch rectangles.

On each of the bottom rectangles, spoon about 1 tablespoon of strawberry jam, leaving a small border around the edges.

Carefully place the remaining rectangles on top of the filled ones and press the edges together.

Brush the tops of the sealed sourdough pop tarts with the lightly beaten egg.

Arrange the prepared pop tarts on a parchment-lined baking sheet and place them in the preheated oven at 375°F. Bake for about 28 minutes.

After baking, carefully transfer the sourdough pop tarts to a wire rack and let them cool for about 10 minutes.

Finally, mix powdered sugar with a splash of water to create a drizzling consistency for the glaze. Drizzle the glaze over the tops.