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

In a microwave-safe bowl, heat the strawberry or raspberry jam for 30 seconds, then stir in the cornstarch and sugar until well combined.

In a large mixing bowl, combine sliced strawberries, blueberries, raspberries, and blackberries. Gently fold in the jam mixture.

Unroll the pie crust onto the baking sheet and flatten it. Spoon the berry mixture into the center, leaving about 1.5 inches of edge.

Carefully fold the edges of the crust over the filling, creating pleats. Sprinkle extra sugar on the crust.

Bake in the preheated oven for 15 to 20 minutes until golden brown and the filling bubbles.

Remove from oven and cool slightly on the baking sheet for about 5 minutes. Serve warm or at room temperature.