In a medium saucepan, combine mixed berries, sugar, and lemon juice. Cook over medium heat for about 5 minutes or until the berries soften and release their juices. Stir in a cornstarch slurry and cook for 1-2 minutes until thickened.

Preheat the oven to 325°F (160°C) and line a muffin tin with paper liners. Combine graham cracker crumbs, sugar, and melted butter in a mixing bowl until the texture resembles wet sand. Press 1 tablespoon into each muffin cup to form a base. Bake for 5 minutes and allow to cool.

In a large mixing bowl, beat softened cream cheese and sugar until smooth. Add sour cream, egg, vanilla extract, and lemon zest (if using). Mix on low until fully combined.

Spoon cheesecake filling into each cooled crust, filling them about ¾ full. Top with cooled berry swirl sauce and swirl gently with a toothpick.

Bake in the preheated oven for 16-18 minutes. Once centers are set but slightly jiggly, remove and cool in the pan. Chill in the fridge for at least 2 hours.