make the topping: In a food processor, combine the flour, sugars, oats, butter and salt and process until the mixture resembles coarse meal.
make the filling : Bring a large pot of water to a boil. Add the peaches and blanch for 1 minute to loosen their skins. Transfer the peaches to a large rimmed baking sheet. When cool enough to handle, peel off the skins and cut the peaches into 1/2-inch dice. Transfer the peaches to a large bowl and sprinkle with the cornstarch. Toss well to coat and let stand for a few minutes.
Preheat the oven to 325°. Butter and sugar twelve 5-ounce ramekins and arrange them on a large, rimmed baking sheet. Spoon the peaches into the ramekins and sprinkle with the topping.
Bake the crisps for about 1 hour, until the filling is bubbling. Remove the ramekins from the oven and increase the temperature to 400°. Bake the crisps on the upper rack for about 8 minutes, until the topping is browned. Serve warm or at room temperature with ice cream, if desired.