Preheat your oven to 375°F (190°C) and line a sheet pan with parchment paper.

Heat the jam in a microwave-safe bowl for about 30 seconds. Stir in corn starch and sugar until smooth.

Add the sliced strawberries, blueberries, raspberries, and blackberries to the jam mixture and fold gently until coated.

Unroll the pie crust on the sheet pan and pour the fruit filling into the center, leaving 1.5 inches around edges.

Fold the crust edges over the filling to create a barrier and sprinkle sugar on the crust.

Bake in the preheated oven for 15-20 minutes until the crust is golden brown and the filling bubbles slightly.

Let the galette cool slightly on a wire rack before serving it warm or at room temperature with ice cream or whipped cream.