In a large Dutch oven, heat 1 Tbsp. oil over medium-high heat. Add half the beef and cook, turning several times until seared. Remove to a plate. Add the remaining 1 Tbsp. oil and brown the remaining beef and remove that to the plate as well.
Add the onion and garlic to the pot and cook, stirring, until the onion is softened. Add the tomato paste and cook, stirring, for another minute. Add the beer, carrots and potato and 1/2 cup of the beef broth. Add the browned beef pieces. Stir to combine. Season with salt, pepper and spices. Bring to a boil, then reduce heat and simmer, covered, for 30 minutes or until beef and vegetables are tender.
After 30 minutes, combine the remaining 1/2 cup beef broth with the cornstarch and add to the pot. Cook uncovered, stirring, until sauce thickens. Spoon the mixture into an 8-inch bottom-diameter cast-iron skillet or baking dish, place on a baking sheet and set aside to cool for at least 15-20 minutes. You don't want to place the puff pastry onto hot filling.
Meanwhile, preheat your oven to 425° F. (non convection/not fan-assisted) with the rack in the centre of the oven. If your puff pastry is in a block, roll one block into a square large enough to cover the top of your skillet with a bit hanging over the edges. If desired, with the other sheet, roll it into a rectangle and cut shapes from it using a leaf-shaped cookie cutter. When the stew has cooled slightly, lay the rolled pastry sheet over the top. In a small bowl, whisk together the egg with 1 Tbsp. of water and brush this mixture over the puff pastry. if using, place the pastry leaves on top and brush those with the egg wash as well.
Place into the oven on top of the baking sheet to catch any bubble-overs and bake for 15 minutes, then lower the oven temperature to 400° F. and bake for an additional 5-10 minutes until golden brown.
Notes
Note 1: You will need 1 sheet of puff pastry for the topping. If desired, you can use a second sheet of puff pastry to cut out shapes to place on top of the pie. I've used a leaf-shaped cookie cutter here, but you could use any shape you like.Be sure to read the notes above this Recipe Card, where I share more detailedtips, variations and substitution suggestions for this recipe!