For the Filling:

• Cook the Vegetables: Heat the olive oil in a large frying pan over medium heat. Add the chopped onion, carrots, and celery. Cook for 8-10 minutes, stirring occasionally, until the vegetables are softened. Stir in the minced garlic and cook for another 2 minutes.
• Brown the Minced Beef: Add the minced beef to the pan and cook for 6-7 minutes, breaking it up with a spoon until browned all over.
• Add Flavor and Simmer: Stir in the tomato purée and Worcestershire sauce. Mix well, then pour in the beef stock and sprinkle with dried thyme. Season with salt and black pepper to taste. Bring to a simmer, reduce the heat, and let it cook gently for 20 minutes, stirring occasionally, until the sauce thickens.

For the Mashed Potato Topping:

• Boil the Potatoes: While the filling simmers, place the potatoes in a large saucepan of salted water. Bring to a boil and cook for 15 minutes, or until tender. Drain the potatoes and return them to the pan.
• Mash the Potatoes: Add the butter, milk, and a pinch of salt and pepper to the potatoes. Mash until smooth and creamy. Stir in the grated cheddar cheese if using.

Assemble and Bake:

• Layer the Pie: Preheat the oven to 200°C (180°C fan) or 400°F. Spoon the beef filling into a large baking dish and spread it out evenly. Carefully spoon the mashed potatoes on top and smooth them out with a spatula or fork. Use the back of the fork to create ridges for extra crispiness.
• Bake the Cottage Pie: Place the dish in the preheated oven and bake for 20-25 minutes, or until the top is golden and the filling is bubbling.
• Serve and Enjoy: Remove from the oven and let it cool slightly before serving. Enjoy this rich and comforting dish on its own or with steamed greens.