Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine the fresh, sliced peaches and blueberries. Sprinkle half of the granulated sugar over the fruits and gently toss them to coat. Allow this mixture to sit for about 10 minutes.

In another bowl, whisk together the remaining sugar, milk, all-purpose flour, eggs, melted butter, vanilla extract, salt, and ground cinnamon (if using). Blend until smooth.

Grease a pie dish with butter or cooking spray. Pour one-third of the custard mixture into the bottom of the pie dish. Layer the prepared fruit evenly on top, then pour the remaining custard mixture over the fruit.

Place the pie in the preheated oven and bake for 45 to 55 minutes until the top is golden brown and the filling bubbles. Test for doneness with a knife.

Once baked, remove the pie from the oven and place it on a wire rack to cool for at least 30 minutes.