01. Heat a drizzle of olive oil in a large pan over medium heat. Add the finely chopped onion and diced carrots, cooking until the vegetables begin to soften, about 5-7 minutes. Then, add the minced garlic and cook for an additional 2 minutes until fragrant.

02. Increase the heat to medium-high, add the minced beef (or lamb) to the pan, and cook until browned, breaking up any lumps with a wooden spoon. This should take about 8-10 minutes. Once the meat is browned, season with salt and pepper to taste.

03. Stir in the tomato paste, Worcestershire sauce, dried thyme, and dried rosemary (if using). Pour in the beef stock, mix well, and bring the mixture to a simmer. Reduce the heat to low, cover, and let it cook for 15-20 minutes, allowing the flavors to meld together. Stir occasionally.

04. After the filling has simmered, stir in the frozen peas and cook for an additional 5 minutes. If the mixture appears too dry, you can add a bit more beef stock or water. Once done, remove from heat and set aside.

05. While the filling is cooking, place the peeled and chopped potatoes in a large pot of salted water. Bring to a boil and cook until the potatoes are tender, about 15-20 minutes. Drain the potatoes and return them to the pot.

06. Add the unsalted butter and milk to the potatoes. Mash until smooth and creamy. Season with salt and pepper to taste. If the mash is too thick, you can add a little more milk to achieve the desired consistency.

07. Preheat your air fryer to 180°C (360°F). Transfer the meat filling into an oven-safe dish that fits your air fryer. Spread the mashed potatoes evenly over the top of the meat filling, smoothing with a spatula.

08. In a small bowl, mix the grated cheddar cheese, breadcrumbs, and melted butter. Sprinkle this mixture evenly over the top of the mashed potatoes. This will create a crispy, cheesy topping.

09. Place the assembled shepherd's pie into the preheated air fryer. Cook at 180°C (360°F) for 15-20 minutes, or until the top is golden brown and crispy. Keep an eye on it to ensure the topping doesn’t burn.

10. Once cooked, carefully remove the dish from the air fryer. If desired, sprinkle with fresh chopped parsley for garnish. Let the shepherd's pie rest for a few minutes before serving.