In a mixing bowl, combine strawberries, blueberries, raspberries, 1/2 cup of sugar, cornstarch, lemon juice, and vanilla extract. Stir gently to combine.

Let the mixture sit for about 10 minutes to allow the berries to release their juices.

Preheat your oven to 375°F (190°C).

Pour the berry mixture into the prepared pie crust, spreading it evenly.

Using a spatula, create swirls in the filling for a decorative effect.

Bake in the preheated oven for 30-35 minutes, or until the filling is bubbly and the crust is golden brown.

While the pie is baking, whip the heavy cream, powdered sugar, and vanilla extract in a mixing bowl until soft peaks form.

Allow the pie to cool for at least 15 minutes before slicing. Serve with a dollop of whipped cream on top.