Prepare the mashed potato topping. Peel the potatoes and cut them into roughly equal 5cm (2 inch) chunks. Place them in a large pot and cover with cold water by about 2.5cm (1 inch). Add a generous pinch of salt.

Bring the potatoes to a boil over high heat, then reduce the heat to medium and simmer for 15-20 minutes, or until they are fork-tender. You should be able to easily pierce them with a fork.

Drain the cooked potatoes thoroughly in a colander. Return the drained potatoes to the warm, empty pot over low heat for 1-2 minutes, shaking the pot occasionally, to allow any excess moisture to evaporate. This is crucial for fluffy mash.

Mash the potatoes using a potato masher or a ricer until smooth. Add the unsalted butter (125g / 1/2 cup + 1 tbsp) and the warm whole milk (125ml / 1/2 cup). Continue mashing until the butter is melted and the milk is fully incorporated, creating a smooth and creamy consistency.

Season the mashed potatoes generously with salt, black pepper, and a pinch of freshly grated nutmeg. Stir well to combine. Cover the pot and set aside. Do not refrigerate.

Preheat your oven to 190°C (375°F).

While the potatoes are cooking, prepare the lamb filling. Heat the olive oil (30ml / 2 tbsp) in a large, heavy-bottomed oven-safe skillet or Dutch oven over medium-high heat.

Add the ground lamb (1kg / 2.2 lb) to the hot skillet. Break it up with a spoon and cook, stirring occasionally, until it is evenly browned, about 8-10 minutes. Drain off any excess fat, leaving about 15ml (1 tbsp) in the pan for flavor.

Add the finely chopped yellow onions (2 medium), diced carrots (2 medium), and diced celery (2 stalks) to the skillet with the lamb. Cook, stirring frequently, until the vegetables have softened, about 5-7 minutes.

Stir in the minced garlic (4 cloves) and cook for another minute until fragrant. Be careful not to burn the garlic.

Sprinkle the all-purpose flour (50g / 1/2 cup) over the lamb and vegetable mixture. Stir well to coat everything and cook for 1-2 minutes to cook out the raw flour taste.

Add the tomato paste (30g / 2 tbsp) and stir it into the mixture, cooking for 1 minute to deepen its flavor.

Pour in the stout beer (330ml / 11.2 fl oz). Scrape the bottom of the pan with your spoon to deglaze, loosening any browned bits stuck to the bottom. Let the stout simmer and reduce slightly for about 2-3 minutes.

Pour in the beef broth (500ml / 2 cups) and add the Worcestershire sauce (15ml / 1 tbsp), dried thyme (5g / 1 tsp), and the bay leaf (1). Stir everything together.

Bring the mixture to a simmer, then reduce the heat to low, cover the skillet, and let it cook gently for 25-30 minutes, or until the gravy has thickened and the flavors have melded. Stir occasionally.

Remove the bay leaf from the lamb mixture. Taste and season generously with salt and black pepper as needed. The filling should be rich and savory.

Remove the skillet from the heat. Spoon the prepared mashed potatoes evenly over the top of the lamb filling, spreading it to the edges to create a seal. Use a fork to create ridges or swirls on the surface of the potatoes; these will crisp up nicely.

Place the skillet in the preheated oven (190°C / 375°F). Bake for 25-30 minutes, or until the potato topping is golden brown and slightly crisp, and the filling is bubbling around the edges.

Once baked, carefully remove the shepherd's pie from the oven. Let it rest for 10-15 minutes before serving. This allows the filling to set slightly, making it easier to serve.

Garnish with fresh chopped parsley (15g / 1/4 cup), if desired, and serve hot.