Butter a 13x9" baking dish with unsalted butter. Season one 1½–2-lb. skinless, boneless turkey breast generously with kosher salt and freshly ground black pepper. Heat 2 Tbsp. extra-virgin olive oil in a large high-sided skillet over medium. Cook turkey until browned, 6–8 minutes per side. Transfer to a cutting board; let rest 10 minutes.

Meanwhile, add 2 Tbsp. water to pan, scraping up any browned bits. Increase heat to medium-high, add 2 Tbsp. unsalted butter and remaining 2 Tbsp. extra-virgin olive oil, and cook until butter is melted. Add 1 large onion, thinly sliced, and 1 large russet potato, scrubbed, cut into ½" pieces, season with kosher salt, and cook, stirring occasionally, until onion is softened and golden brown around edges, about 5 minutes. Add 20 oz. frozen mixed vegetables and 2 garlic cloves, finely chopped; season with kosher salt and pepper. Cook, stirring occasionally, until heated through, 5–7 minutes.

Meanwhile, chop turkey into ½" pieces. Whisk together ¼ cup all-purpose flour and 2 cups water in a large measuring glass or small bowl.

Reduce heat to medium and pour flour mixture into pan; stir vigorously until no lumps remain. If mixture seizes, add more water ¼-cupful at a time, whisking until sauce is smooth. Bring to a simmer and cook, stirring, until thickened, about 3 minutes. Remove pan from heat, add 4 oz. American cheese (4–6 slices), finely chopped, and stir until melted. Transfer turkey and any accumulated juices to pan, add ⅓ cup chopped parsley, and stir to combine. Scrape into prepared baking dish.

Preheat oven to 400°. Unfold 1 sheet frozen puff pastry (one 14-oz. package or half of a 17.3-oz. package), thawed, and gently roll out on a lightly floured surface to smooth out creases and make slightly larger than baking dish if needed. Drape over dish and press gently so it touches filling. Brush with 1 large egg, beaten to blend, and sprinkle with flaky sea salt. Using a small knife, make at least four 1–2" slits in pastry for venting as desired.

Bake pot pie until pastry is golden brown, 40–45 minutes. Let cool 20–25 minutes before serving. Editor’s note: This recipe for turkey pot pie with puff pastry was first printed in our November 2022 issue; it has been updated for style. Head this way for more of our best recipes using puff pastry →