2 tablespoons (28 grams) unsalted butter, softened¾ cup (86 grams) finely diced celery⅓ cup (37 grams) finely diced sweet onion4 cups (500 grams) all-purpose flour1 tablespoon (9 grams) kosher salt4 teaspoons (20 grams) baking powder1 tablespoon (12 grams) granulated sugar1½ teaspoons (3 grams) poultry seasoning½ teaspoon (1 gram) garlic powder½ teaspoon (1 gram) ground black pepper1 cup (227 grams) cold unsalted butter, cubed½ cup (57 grams) shredded Gruyère cheese1¼ cups (300 grams) cold whole buttermilk1 large egg (50 grams)1 tablespoon (15 grams) waterFlaked sea salt
Line a rimmed baking sheet with parchment paper.
In a medium skillet, melt softened butter over medium-high heat. Add celery and onion; cook, stirring frequently, until tender and fragrant, 5 to 6 minutes. Remove from heat; let cool completely.
In a large bowl, whisk together flour, kosher salt, baking powder, sugar, poultry seasoning, garlic powder, and pepper. Add cold butter, tossing to coat. Using a pastry blender or your hands, cut in butter until mixture is crumbly and butter pieces are pea-size. Stir in cooled vegetable mixture and cheese. Add cold buttermilk; stir with a fork until a shaggy dough forms. (Do not overmix.)
Turn out dough onto a lightly floured surface. (Dough will be crumbly and almost dry.) Pat dough into a square about 1¼ inches thick. Using a bench scraper or sharp knife, cut into fourths. Stack fourths on top of each other, and pat down to 1¼- inch thickness. Repeat cutting and stacking 3 more times. (Dough may seem too shaggy at first but will come together and be easier to work with as you go.)
Roll dough into a 1¼-inch-thick square. Using a 2¼-inch round cutter dipped in flour, cut 12 rounds from dough, rerolling scraps once. Place 1 inch apart on prepared pan. Freeze for 15 minutes.
Preheat oven to 425°F (220°C).
In a small bowl, whisk together egg and 1 tablespoon (15 grams) water; brush onto biscuits. Sprinkle with sea salt.
Bake until golden brown, 18 to 22 minutes. Let cool on pan for 5 minutes. Best served warm. Store in an airtight container for up to 3 days.
Pro Tip:
Biscuits can be prepared through step 5, frozen until firm, and then transferred to a heavy-duty resealable bag to freeze for up to 1 month. Bake as directed from frozen, adding about 5 minutes to bake time.