Instructions

1. Make the crust: Pulse flour, salt, mustard, and lemon zest together in a food processor until combined. Add butter and pulse until pea-size crumbles form. Continue to pulse, sprinkling in ice-cold water until dough comes together. Transfer dough to a lightly floured surface and form into a ball. Form 2⁄3 of dough into a flat disk; wrap in plastic wrap. Repeat with remaining dough; chill 1 hour.
2. Make the filling: Heat half the oil in an 8-qt. saucepan over medium-high heat. Add chopped onion; cook, stirring occasionally, until golden, 5–7 minutes. Transfer to a bowl; set aside. Add remaining oil to pan, and season meats with salt and pepper. Working in batches, cook meats, turning as needed, until browned, 25–30 minutes. Return all meat to pan. Add halved onion, stock, thyme sprigs, and bay leaf; bring to a boil. Reduce heat to medium-low; cook, covered, until meat is very tender, about 1 hour. Using a slotted spoon, transfer meat to a plate and let cool. Simmer liquid until reduced to 2 cups, about 40 minutes. Strain into a large bowl, discarding solids; set aside to cool. When meat is cool enough to handle, shred and add to reduced cooking liquid. Add reserved chopped onion, plus chopped thyme, flour, vinegar, mustard, half the beaten eggs, and salt and pepper; stir to combine.
3. Assemble and bake the pie: Heat oven to 350°. Grease a 9" x 13" baking dish with butter; set aside. On a lightly floured surface, roll larger disk of dough into a 13" x 17" rectangle about 1⁄4" thick. Fit dough into greased pan, letting excess dough hang over edges of pan. Spread filling evenly over bottom of crust. Roll remaining smaller disk of dough into a 11" x 15" rectangle about 1⁄4" thick; transfer to top of dish. Trim excess dough and fold inward, pinching and crimping edges to seal. Using a knife, cut 4 slits in top crust and brush with remaining beaten eggs; bake until golden brown and cooked through, about 1 hour. Let pie cool slightly before slicing and serving.