Preheat your oven to 400°F (200°C).

On a clean surface, unroll the refrigerated pie crust and roll it into a 12-inch circle.

Transfer the crust to a parchment-lined baking sheet and let it sit.

In a large bowl, combine the sliced plums, 3 tablespoons of coarse sugar, and cornstarch. Gently mix until coated.

Arrange the plum mixture on the prepared crust, leaving a 2-inch border around the edges.

Fold the edges of the crust over the fruit, creating pleats as you go.

Beat the egg white with water and brush over the folded crust, then sprinkle with the remaining teaspoon of sugar.

Bake the tart for 20-25 minutes, watching for a golden brown crust.

Allow the tart to cool on a wire rack for at least 10 minutes before serving.