Preheat the oven to 375°F. Rinse and pit the peaches and evenly slice the peaches.
Add the sliced peaches, blueberries, sugar, cornstarch, vanilla, cinnamon and lemon juice to a 9×13 casserole dish or a 12-inch cast iron and mix until well combined. (You can mix it all in a bowl, then transfer it to the casserole dish, along with the juices.)
In a bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon.
Add the cold, cubed butter to the dry mixture. Using a pastry cutter, two forks, or your fingertips, work the butter into the flour until the mixture resembles coarse crumbs. (The butter pieces should be about the size of small peas after mixing.)
Pour in the cold milk and gently mix until just combined, the dough will be shaggy and lumpy. (Do NOT overmix the batter.)
Using a spoon or a cookie scoop, drop dollops of the biscuit dough evenly over the fruit filling.
Bake at 375°F for about 45-50 minutes, or until the biscuit topping is golden and the fruit is bubbly.
Remove from the oven and allow it to cool for 5-10 minutes.
Serve warm with a scoop of vanilla ice cream.