Heat oil in a large pot or Dutch oven over medium-high heat.
Add beef and brown on all sides. Season with salt.
Add chopped onion and cook until softened, about 5 minutes.
Stir in minced garlic and cook for 1 minute until fragrant.
Add tomato paste, black pepper, and white pepper (if using). Cook for 1-2 minutes, stirring.
Sprinkle flour over the beef and stir to coat. Cook for 1 minute.
Pour in beef stock, Worcestershire sauce, soy sauce, Dijon mustard (if using), and sugar. Add bay leaf.
Bring to a simmer, then reduce heat, cover, and cook for 1.5-2 hours, or until beef is tender.
Remove bay leaf. Stir in cornflour slurry and cook until sauce thickens.
Preheat oven to 200°C (400°F).
Grease a baking tray.
Line the tray with two sheets of puff pastry, overlapping slightly.
Pour the pepper steak filling into the pastry-lined tray.
Cover with the remaining sheet of puff pastry.
Trim excess pastry and crimp edges to seal.
Brush the top with beaten egg.
Bake for 20-25 minutes, or until pastry is golden brown and puffed.
Let stand for a few minutes before serving.
