Cornish Pasties

FOR THE CRUST: Whisk sour cream and egg together in small bowl. Process flour and salt in food processor until combined, about 3 seconds. Add butter and pulse until only pea-size pieces remain, about 10 pulses. Add half of sour cream mixture and pulse until combined, about 5 pulses. Add remaining sour cream mixture and pulse until dough begins to form, about 15 pulses.

Transfer mixture to lightly floured counter and knead briefly until dough comes together. Form dough into 6-inch disk, wrap tightly in plastic wrap, and refrigerate for 30 minutes. (Dough can be refrigerated for up to 24 hours; let chilled dough sit on counter for 15 minutes to soften before rolling.)

FOR THE FILLING: Melt butter in 10-inch skillet over medium heat. Add onion and 1/4 teaspoon salt and cook until softened, about 5 minutes. Add thyme and garlic and cook until fragrant, about 30 seconds. Let cool slightly, about 5 minutes. Combine cooled onion mixture, steak, potatoes, rutabaga, 2 teaspoons salt, and 3/4 teaspoon pepper in bowl. Add flour and toss to coat.

Adjust oven rack to upper-middle position and heat oven to 375 degrees. Line rimmed baking sheet with parchment paper. Remove dough from refrigerator and cut into 6 equal pieces (about 5 ounces each); cover with plastic wrap. Divide filling into 6 equal portions, about 1 heaping cup each.

Working with 1 piece of dough at a time, roll into 10 by 8-inch oval (about 1/8 inch thick) on lightly floured counter. Place 1 portion filling in center of dough. Moisten edges of dough with water, then fold narrow end of oval over filling to form half-moon shape. Press dough around filling to adhere.

Trim any ragged edges, then crimp edges with fork to seal; transfer to prepared sheet. (For more decorative edge, trim any ragged edges and, starting at one end, pinch and slightly twist dough diagonally across seam between your thumb and index finger. Continue pinching and twisting dough around seam.) Repeat with remaining dough and filling.

Using paring knife, cut 1-inch vent hole on top of each pasty. Whisk egg and 2 teaspoons water in bowl. Brush pasties with egg wash. Bake until crust is golden brown and filling is bubbling up through vent hole, about 45 minutes, rotating sheet halfway through baking. Transfer pasties to wire rack and let cool for 10 minutes before serving.

TO MAKE AHEAD: Pasties can be prepared through step 6, then frozen on baking sheet. Once frozen, pasties can be stored in zipper-lock bag for up to 1 month. To cook from frozen, bake at 350 degrees for 1 hour 5 minutes to 1 hour 10 minutes.