For the filling: Butter a 7-by-3-inch round baking dish and set aside. In a large bowl, toss together the sliced peaches, blueberries, lemon juice, sugar, arrowroot, vanilla and salt. Pour the mixture into the prepared baking dish.
For the topping: In a separate bowl, mix together the oats, flour, sugar, cinnamon and salt. Using your hands, work the butter into the flour mixture until there are no longer any powdery ingredients at the bottom of the bowl. Break apart large pieces of the topping mixture and place on top of the fruit until the fruit mixture is almost entirely covered. Bake until the topping is golden brown and crispy and the fruit mixture is bubbling, about 40 minutes. Let cool for at least 20 minutes. Serve with the ice cream.