For your top crust, in a bowl, stir together your flour and salt. Rub your cold butter into the mixture until it resembles coarse crumbles
Add in your cold water, stir together, and then compress the dough into a ball. See notes below if you have a food processor!
Place your dough ball between two large pieces of parchment paper, and roll to ⅛” thickness. Make sure your dough is an even thickness throughout
Place your rolled crust in the fridge to chill while you make your filling
Heat a 10-inch oven-safe skillet on the stove over medium-high heat
Add your olive oil to the pot along with your diced carrots, onion, mushrooms, celery, salt, and ground pepper. Sauté for 7- 10 minutes until cooked down and the mushrooms and onions start to get some color on them
Add in your garlic and cook an additional 2 minutes, until fragrant
Stir in your herbs and cook for an additional 1-2 minutes
Add in your prosecco and use it to scrape any bits off the bottom of the pan. Cook until the prosecco is absorbed into the veggie mix
Add in your 2 ½ tablespoons of flour, stirring to evenly coat your veggies, then add your butter. Combine until you have an almost paste-like mixture
Add your diced parboiled potatoes* and stir to combine
Pour in your broth, and stir to bring everything together before bringing to a boil. Reduce heat and simmer for 20 minutes, until the mixture has thickened. Be sure to stir occasionally, scraping any bits off the bottom
Add in your milk/cream about 15 minutes in, once it’s thickened slightly. Stir and continue cooking until it’s thickened to a stew-like texture
Remove from the heat and stir in your peas and corn
Pour into your pie dish, spread evenly, and cool slightly for 10 minutes
Preheat your oven to 400°F
Sprinkle 1 tsp flour over the top of your filling. This will help prevent the underside of your top crust from being soggy
Cover with your rolled-out top crust, folding in the edges and crimping. Cut a few slits into the dough from the center to vent the pie as it bakes
In a bowl, whisk together your egg and a splash of water or milk. Brush across the top of your pie crust and sprinkle with flaky salt and sesame seeds if desired
Place a baking sheet on the rack below your pie, to catch any drips
Bake for 40 -45 minutes until the crust is golden brown, rotating halfway through
Allow to cool for at least 15 minutes before slicing and serving
Enjoy!