First, prepare the filling. In a large bowl, combine the lemon juice, sugar, and cornstarch. Mix until smooth.
Add in your sliced peaches (peeled if you'd like) and blackberries, and toss to combine.
Next, grab your store-bought pie crust, and unroll it. Place it on a baking sheet lined with a silicone liner or parchment paper.
Heap the blackberry peach filling in the center of the crust, leaving any excess liquid behind.
Fold and crimp the edges of the pie over to create the crust. Brush the crust with nondairy milk, and sprinkle with Turbinado sugar.
Chill the galette for 30+ minutes until solid. Meanwhile, preheat oven to 400F.
When galette is chilled, bake for 35-40 minutes until golden brown and flaky, and the filling is bubbling.
Serve warm with ice cream or coconut whip!