Beef: Heat 1 tbsp olive oil in a heavy-based pot over high heat. Brown the mince then remove and set aside. Dice 1 onion, 2 carrots and 2 stalks celery and add to the same pot. Cook over medium heat for 4–5 minutes until softened. Add 2 tsp crushed garlic and cook for a further minute. Slice 200g mushrooms and add to the pot along with 1 tbsp tomato paste, stirring to coat. Return the beef to the pot, then add 1 tin chopped tomatoes, 1 cup beef stock, 1 tsp Worcestershire sauce, 1 tsp salt, 1 tsp pepper and 1 tsp dried thyme. Bring to a simmer, then cover and cook on low for 30 minutes until the sauce has thickened.
Pie: Preheat the oven to 200°C. Spoon the filling into a large ovenproof dish or pie dish. Scatter 1 cup grated colby cheese over the top. Cut the puff pastry sheet to fit the dish and lay over the top, pressing the edges down. Brush with 1 beaten egg. Bake for 15 minutes, or until the pastry is golden and puffed.
Serve: Serve straight from the dish at the table.