Need an all-around great winter dinner? Beef comingles with mushrooms, potatoes, carrots, and peas in this pot pie that's toothsome yet tender, with just the right amount of vegetal sweetness.
Place potatoes in a small saucepan; add water to cover. Bring to a boil over high. Reduce heat to medium; simmer until just tender, 8 to 10 minutes. Drain.
Heat 1 tablespoon of the oil in a large skillet over high. Add beef; cook, stirring occasionally, until browned on all sides, 5 to 6 minutes. Transfer to a plate; sprinkle with 1/2 teaspoon of the salt and 1/2 teaspoon of the pepper.
Add sliced mushroom caps and remaining 1 tablespoon oil to skillet; cook over high, stirring often, until lightly browned, 4 to 5 minutes. Add carrots and thyme. Reduce heat to medium-high; cook, stirring often, until mushrooms have released their liquid, 3 to 4 minutes.
Whisk together flour and 1 cup of the stock in a small bowl until smooth. Add remaining 3 cups stock to mushroom mixture in skillet; bring to a boil over medium-high, stirring occasionally. Stir in flour-stock mixture. Reduce heat to medium; simmer, stirring often, until carrots are just tender and sauce has thickened, 6 to 8 minutes. Transfer mixture to a large heatproof bowl; stir in potatoes, beef, peas, and remaining 1 1/2 teaspoons salt and 1/4 teaspoon pepper.
Preheat oven to 400°F. Coat an 8-inch square glass or ceramic baking dish with cooking spray. Roll 1 of the piecrusts into an 11-inch circle; fit inside prepared baking dish, pressing gently into corners and allowing overhang on sides of dish. Spoon beef mixture over piecrust in dish; top with remaining piecrust. Press together top and bottom piecrust edges to seal, incorporating any piecrust overhang into pressed edges. Cut 5 to 6 slits on top of piecrust in a pinwheel pattern. Whisk together egg and the 2 tablespoons water in a small bowl; brush onto piecrust.
Place baking dish on a rimmed baking sheet. Bake in preheated oven until filling is bubbling and top is lightly browned, 30 to 35 minutes. Let cool 15 minutes; serve.