2 TBSP olive oil
1 1/2 lbs mushrooms (such as cremini or button), halved, or quartered if large
4 carrots, cut into 1/2-inch pieces
3 celery stalks, sliced 1/4 inch thick
1 medium onion, chopped
1/2 tsp dried thyme
kosher salt and black pepper
1/3 cup all-purpose flour
1 1/2 cups vegetable broth
1 cup frozen peas
1 sheet puff pastry (half a 17.3-ounce package), thawed
Heat oven to 400 deg. Heat 2 TBSP of the oil in a large skillet over medium-high heat. Add the mushrooms, carrots, celery, onion, thyme, ½ tsp salt, and ¼ tsp pepper. Cook, stirring occasionally, until the vegetables are tender, 12 to 15 minutes. Add the flour and cook, stirring, for 30 seconds. Add the broth and peas; bring to a boil. Transfer the mushroom mixture into an 8-inch square baking dish. Lay the pastry on top and cut several vents in it. Place the baking dish on a rimmed baking sheet and bake until the crust is golden, 25 to 30 minutes. Let rest 15 minutes before serving.
Tip: The potpie can be assembled up to 1 day in advance. Cool the mushroom mixture before transferring it to the baking dish and topping with the pastry; refrigerate, covered. Bake as directed, adding 5 to 10 minutes to the cooking time.