Make the pie dough. Whisk together the flour and salt in a large bowl. Cut in the butter with a fork, a pastry cutter, 2 butter knives, or your fingertips. It should look crumbly, with some pieces of butter the size of small peas. Drizzle the water in a little at a time, mixing as you go (you can use a fork to mix, but mixing with your fingertips works best here). Add just enough water so it comes together to form a ball of dough (you may need more or less water).
Chill the pie dough. Shape the dough into a ball, flatten it slightly into a disk, wrap it in plastic wrap, and refrigerate until chilled, at least 30 minutes (or up to 3 days).
For the Beef Filling:
Brown the beef and soften the vegetables. Heat a 12-inch cast-iron skillet over medium-high heat. Add the ground beef, spreading it out in an even layer, and cook (uncovered) until browned, about 5 minutes, breaking up the meat into small crumbles as it cooks. Stir in the onion, carrot, and potato, cover the skillet, and cook until the veggies are starting to soften, about 5 to 8 minutes, stirring occasionally.
Add the aromatics and thickener. Turn the heat down to medium. Add the garlic and rosemary and cook 1 minute, stirring frequently. Sprinkle the flour onto the beef mixture, and stir it to incorporate; cook 1 minute, stirring constantly.
Add the liquid and seasonings. Stir in the beef stock and use a wooden spoon to scrape up the brown bits from the bottom. Stir in the tomato paste, Worcestershire, Dijon, salt, and pepper. Bring up to a boil, and cook 3 minutes, stirring frequently. Remove from the heat and stir in the peas.
To Bake:
Prep. Preheat the oven to 400F.
Roll out the crust. Place the dough onto a floured work surface. Working from the center out, roll the dough out to a circle about 12 to 13 inches in diameter. Working quickly but carefully to avoid the butter melting as much as possible, fold the dough over onto itself, pick it up, and unfold it on top of the beef filling, tucking the outer portion under the filling or crimping if desired.
Egg wash and vent the top. Lightly brush the dough with egg wash. Cut a few slits in the center of the top so steam can escape.
Bake. Bake until the crust is golden and the filling is bubbling out of it, about 34 to 38 minutes. Let the pot pie cool for 10 to 15 minutes before serving.
Video
Notes
Time-Saving Crust Options: To simplify, you can use a store-bought pie crust, crescent roll dough, biscuit dough, or puff pastry. Another option (similar to shepherd's pie) is to top this casserole with mashed potatoes.
Ground Beef: If you use ground beef that's leaner than that, add 2 tablespoons of olive oil or avocado oil. On the other hand, if you only have ground beef that's fattier than that, use it and drain off all but 2 tablespoons of the rendered fat.
Potatoes: I like to use yellow potatoes (such as Yukon Gold) here for two reasons: 1) they don't break down/disintegrate in the filling the way that starchy potatoes (such as Russet) can, and 2) they have a thin skin so there's no need to peel them, which is important because the peel holds the most nutrition.
Storage: After cooling, store leftovers in an airtight container in the fridge for up to 4 days or in the freezer for up to 3 months.