Prep the pan. Line a quarter-sheet pan with parchment, leaving overhang for easy lifting.
Heat oven to 400°F (200°C).
Make the filling. In a saucepan, combine strawberries, sugar, lemon juice, cornstarch, vanilla, and salt. Cook over medium heat, stirring, until berries release juice and the mixture thickens and bubbles, 5–8 minutes. Cool to room temp.
It should be jammy, not runny.
Roll the dough. On a lightly floured surface, roll one crust into a rectangle to fit the pan with a little overhang. Transfer to the pan. Roll the second crust to a matching rectangle for the top.
Assemble. Brush a 1/2-inch border of egg wash around the bottom crust.
Spread the cooled strawberry filling inside the border in an even layer.
Seal it. Place the top crust over the filling. Press edges to seal, then crimp with a fork. Trim excess if needed.
Prick the top all over with a fork to vent.
Bake. Brush the top with remaining egg wash. Bake 22–28 minutes until golden brown and crisp. If edges brown too fast, tent loosely with foil.
Cool completely. Let it cool in the pan 45–60 minutes.
Warm crust will melt the glaze, so patience helps.
Glaze. Whisk powdered sugar, milk, vanilla, jam (if using), and a pinch of salt until smooth and spreadable. Adjust with a few drops more milk if too thick. Spread over the cooled tart.
Add sprinkles or strawberry crumbs.
Set and slice. Let the glaze set 15–20 minutes. Lift out using the parchment and slice into 12–16 squares.