Pour the berries into a medium saucepan, keeping frozen berries unthawed to preserve texture.

In a small bowl, whisk together cornstarch, sugar, and salt until the cornstarch is lump-free.

Pour the cornstarch mixture over the berries and stir gently to coat evenly.

Add lemon juice and vanilla extract to the berries and stir to combine.

Place the saucepan over medium heat and bring to a gentle simmer, stirring occasionally.

Once simmering, let the compote bubble gently for 6 to 8 minutes, stirring every 2 minutes, until it thickens and becomes glossy.

Use a spoon to gently mash some of the berries against the side of the pan, leaving some whole for texture.

Remove the compote from heat and allow it to cool in the pan for at least 10 minutes; it will thicken further as it cools.

Taste the cooled compote and adjust with additional lemon juice or sugar if necessary before serving or storing.