Preheat the oven to 200°C (400°F). Peel and cut the Floury Potatoes into even chunks and place them in a large pot of boiling water seasoned with Salt; simmer until very tender, about 15–20 minutes.
While the potatoes cook, heat a tablespoon of Olive Oil in a large, ovenproof skillet or frying pan over medium heat. Add the chopped Onion, Carrot and Celery and sauté until softened, about 6–8 minutes.
Stir in the minced Garlic and cook for 30–60 seconds until fragrant, then add the Lamb Mince and brown well, breaking it up with a spoon. Season with Salt and freshly ground Black Pepper.
Sprinkle in the Plain Flour and cook for 1 minute to remove the raw taste, stirring to coat the meat and vegetables.
Mix in the Tomato Paste, a splash of Worcestershire Sauce, the Thyme Leaves, chopped Rosemary and the Bay Leaf. Pour in the Stock, bring to a gentle simmer and cook until the mixture thickens into a rich gravy, about 8–10 minutes. Taste and adjust seasoning.
Stir the Frozen Peas into the meat mixture just to heat through, then remove the pan from the heat and discard the bay leaf if desired.
Drain the cooked potatoes and return them to the hot pot. Mash with Unsalted Butter and warm Milk until smooth and creamy; season with additional Salt and Black Pepper to taste.
Spoon the lamb mixture into an ovenproof dish (or leave in the ovenproof skillet), then spread the mashed potatoes over the top, smoothing or creating peaks with a fork for extra crispiness. Dot with a little extra butter if you like.
Bake in the preheated oven for 20–25 minutes until the top is golden and the filling is bubbling around the edges. For a deeper crust, place under a hot grill for 2–4 minutes — watch closely to avoid burning.
Let the pie rest for 5 minutes before serving so the filling sets slightly. Serve hot and enjoy.
