Pulse the flour, salt, sugar and zest together in a food processor. Pulse in the cubed butter until mixture resembles a coarse meal and the largest bits of butter are the size of large peas. Pulse in the sour cream and 3-4 tablespoons water. Dough should just stick together when squeezed in your palm. Pour mixture out onto a floured surface to knead the mixture into a rough, craggy ball. Wrap in plastic and flatten into a disc. Chill in the fridge for 1 hour or up to 48 hours, or you can quick-firm this in the freezer for 15 minutes.
Preheat oven to 400 degrees. If skinning the peaches, prepare an ice bath. Bring a pot of water to a boil, then blanch peaches just until their skin bursts, about 20-30 seconds. Transfer to ice bath and then remove skins. Pit and slice peaches. In a large bowl, toss peaches with the remaining filling ingredients.
Once dough has chilled, place onto a floured work surface. Roll to about 1/4" thick, then transfer dough to a baking sheet. Pour fruit filling into the center of the dough and fold up edges to contain the filling.
In a small dish, beat a egg with a tablespoon of water. Brush this over crust, then sprinkle with raw sugar. Bake crostata for about 40-45 minutes, until golden brown and bubbly. If juices spill out, don't panic!
Allow to cool for about 10 minutes before slicing. Serve with whipped cream or ice cream.