In a large bowl, mix jam with corn starch until smooth.
Gently add all of the berries and fold into the jam mixture. Try not to stir too much, or the berries will break down. Set berries aside.
Crostata Crust
Mix flour, sugar and salt together in a food processor or blender and pulse.
Add slices of cold butter a little at a time and pulse about 10 times until the dough is crumbly.
While the food processor is running, add the ice water through the opening all at the same time.
Pulse until dough makes an irregular ball, then remove and roll out to fit a 12 inch cast iron skillet.
Putting It Together
Spray 12" cast iron skillet with baking spray.
Place rolled pie crust into skillet, leaving 2 inches over the edges of the skillet.
Pour berry mixture into middle of pie crust.
Roll edges of crust inward, leaving a large area open in the middle. Crimp crust around outside of berries.
Brush edges of crust with butter and sprinkle with sugar.
Bake crostata at 450 degrees for 30 minutes, or until crust is golden brown.
Take out of oven and cool until crostata is room temperature before slicing.
Optional Crust
If desired, use two refrigerated pie crusts and roll them into a larger crust that will fit in a 12" cast iron skillet, with at least 2 inches of overlap on all sides.
Notes
Some of the filling may seep out into the bottom of the skillet. No worries! This is part of the rustic look of this dish.
Let crostata cool to room temperature after baking, then serve with a scoop of vanilla bean ice cream if desired.