Soup in the palm of your hand — why not? The trick to making this tasty finger food lies in the consistency of its filling. Let the onion mixture cook down until it gives off very little moisture before crimping it into the pastry dough.
1 egg beaten with 1 Tablespoon water, for egg wash
Cut butter into ½-inch cubes and keep cold in the refrigerator.
In a large bowl, whisk to combine flour and salt. Add butter, toss to coat in flour, then use your fingers or a pastry blender to cut the butter into pea sized pieces.
Add 6 tablespoons of the cold water mixture and stir until a shaggy dough forms. Bring the dough together with your hands, adding a few more tablespoons of water if still crumbly. Turn dough out onto a floured work surface, divide in half and pat into 2 fat discs. Wrap in plastic wrap and refrigerate for at least 2 hours.
Heat butter in a large skillet over medium heat. Add onions, cover and cook over medium heat, tossing every so often, until softened, about 10 minutes.
Reduce heat to medium-low. Season with salt and pepper and cook, stirring occasionally, until onions are deeply golden and very soft, about 45 minutes.
Add sherry and cook until mixture is mostly dry. Stir in flour and cook for 2 minutes until combined. Stir in beef stock and cook, stirring constantly until thickened, about 5 minutes. Remove from heat and stir in shredded beef, thyme and tarragon. Transfer to a large bowl and let cool.
On a lightly floured surface, roll one disk of pastry dough out to ⅛-inch thick. Use a 4-inch round cookie cutter to cut out 16 rounds, rerolling scraps as needed. Repeat with remaining disk of dough.
Scoop 1 tablespoon of onion mixture into the center of a pastry round. Top with 1 tablespoons of cheese. Brush outer edge with egg wash, top with another pastry round and gently press to seal. Use a fork to crimp the edges and carefully transfer to a parchment-paper lined baking sheet. Repeat to make 7 more hand pies. Freeze for at least 30 minutes.
When ready to bake the pies, preheat oven to 400° F.
Brush tops with egg wash and sprinkle with flaky salt. Cut 3 small steam vents in the top. Bake for 25-30 minutes, or until pies are golden brown. Let cool on the baking sheet. Serve warm.