Preheat oven to 350°F. Lightly grease a 9x13-inch baking dish.
Add the 5 cups diced peaches and 2 cups blueberries to the baking dish.
Sprinkle ⅓ cup granulated sugar, 2 Tablespoons cornstarch, 1 Tablespoon lemon juice, 1 teaspoon vanilla extract, ½ teaspoon ground cinnamon and a Pinch of salt over the fruit. Gently toss until evenly coated, then spread the fruit into an even layer.
In a medium bowl, combine the 1 cup old-fashioned rolled oats, 1 cup all-purpose flour, ¾ cup packed brown sugar, ½ teaspoon ground cinnamon, and ¼ teaspoon salt.
Pour ½ cup unsalted butter, melted over the oat mixture. Stir with a fork until everything is evenly moistened and coarse crumbs form.
Sprinkle the crisp topping evenly over the fruit. Do not pack it down.
Bake for 45 minutes, or until the fruit is bubbling around the edges and the topping is golden brown.
Remove from the oven and cool for at least 45 minutes before serving. The fruit filling will continue to thicken as it cools.Serve warm, with vanilla ice cream if desired.