In wide bowl, or in the bowl of a food processor, mix the flour and salt. Cut in the cold butter until it resembles small peas.
In a separate bowl, whisk together the sour cream or greek yogurt, vinegar and water. Add it to the flour- butter mixture and blend until a craggy dough forms. Form it into a flat disc, wrap in plastic and refrigerate at least 1 hour or up to 2 days.
Lightly flour a clean work surface. Divide the dough in four. Roll out each quarter into rounds that will cover your baking dish with a 1-inch overhang. Whisk the egg with a teaspoon of water, and brush the egg wash over the dough. Cut vents for steam. Sprinkle top of pies with flaky sea salt and parm (optional). Bake pies in a preheated 375 degrees F oven for 30-35 minutes until the crusts are golden and the filling is bubbling.