Prepare the crust first: Place flour, sugar and salt in a food processor and pulse a couple times to combine. Add the cold, cubed butter and pulse until no large chunks of butter remain.
Slowly stream in the cold buttermilk until the dough comes together. At first, it may look dry and crumbly but continue running the food processor, it will be enough to bind the dough and it will come together!
Turn dough out onto a lightly floured surface and gently knead/form dough until it comes together. Divide dough into 2 equal pieces. Form into a semi-flat thick oval shape, wrap in plastic wrap and place in the fridge for at least 30 minutes.
In a large Dutch over or large, deep pan melt the butter. Add the onions, carrots, celery and garlic.
Saute over medium heat until fragrant and soft, about 8 minutes. Season with salt and pepper.
Add flour and stir to coat all of the vegetables.
Add diced ham and stir to combine.
Add broth and milk and bring to a simmer, cook until a thick creamy consistency, about 5 minutes.
Add peas, apple cider vinegar and herbs. Stir to combine and remove from heat to allow to cool slightly while you prepare the crust.
Preheat oven to 375°F.
Roll out one of the disks of dough. Roll into a large circle (about 12 inches) on a lightly floured surface and carefully transfer to a 9” pie dish. (I fold the dough in half, then in half again and unfold onto pie dish, see video).
Trim any overhang that’s long or fold under and pinch to form a thick crust, crimp as desired. Dough is flexible, you can’t really mess it up. And if you do, it looks rustic!
Blind bake for 8-10 minutes. Line dough with parchment paper & fill with pie weights to help keep the edges from sagging as crust bakes*
Spoon the filling onto the crust.
Roll the second disk of pie dough out on a floured surface and carefully place over the pie filling. Trim overhang, seal edges with a fork or crimp edges with your fingers.
Use a small knife to make a couple small slits in the center of the top crust (to allow steam to escape while it bakes).
Use a pastry brush to brush the crust and edges with a beaten egg.
Bake for 35-40 minutes or until crust is golden brown. Use foil on the edges or a pie crust shield to protect edges from browning too much, if necessary.
Allow to rest for 15 minutes before slicing and serving.