The best shepherd's pie

Ingredients:

Topping

• 1 kg potatoes, peeled and cut into 4 pieces (about 4 medium tubers)
• 6 tablespoons unsalted butter
• 1/4 of a small head of white cabbage, finely shredded (about 170g or 2 cups)
• 1 leek (white and light green parts only), chopped
• 2 cloves garlic, chopped
• 3/4 cup heavy cream
• 1 cup grated sharp cheddar (about 80 g)
• 3 large egg yolks

Filling

• 2 tsp extra-virgin olive oil
• 3 medium carrots, peeled and thinly sliced
• 1 medium onion, diced
• 1 stalk celery, thinly sliced
• 2 cloves garlic, chopped
• 2 tsp fresh thyme leaves, chopped
• 1/4 cup tomato paste
• 1 kg of ground lamb
• 1 cup frozen green peas, thawed
• 1/4 cup flour
• 1.5 cups lightly salted chicken broth
• 1 tbsp Worcestershire sauce

Preparation:

1. Preheat oven to 190°C.
2. Topping:
   
   Place the potatoes in a large saucepan and cover with cold water. Sprinkle with 2 teaspoons of salt. Bring to a boil and simmer until tender, about 20 minutes. Drain and set the saucepan aside.
3. Meanwhile, prepare the filling.:
   
   Heat olive oil in a large nonstick skillet over medium heat. Add carrots, onion, celery, garlic, thyme, and 2 teaspoons salt. Cook, stirring occasionally, until the vegetables are soft and tender, about 15 minutes.
4. Add the tomato paste and cook until it turns brick red, about 8 minutes. Add the lamb, 1.5 teaspoons of salt, and a pinch of freshly ground black pepper. Cook, breaking up the mince with a wooden spoon, until no pink meat remains, about 7 minutes.
5. Stir in the green peas. Stir in the flour until fully incorporated, about 1 minute. Add the broth and Worcestershire sauce and simmer until the liquid thickens, 3-5 minutes.
6. In a large saucepan over medium heat, melt the butter. Add the cabbage and leek and cook until the cabbage is tender, about 10 minutes. Add the garlic and cook until fragrant, 1 minute. Add the heavy cream, 1 tablespoon of salt, and some freshly ground black pepper. Cook for about 1 minute. Return the potatoes to the saucepan with the cabbage and add the cheese. Mash everything with a potato masher or a wooden spoon. Remove from the heat and stir in the egg yolks.
7. Transfer the filling to a 22x32 cm baking dish. Spread the topping on top, leaving swirls on the surface. Press the potato topping right up to the sides of the dish so the filling is hidden.
8. Bake until the filling is heated through and the top is golden brown, about 35 minutes. Let the pie rest for 15 minutes and serve.