Heat olive oil in a large skillet over medium heat. Add chopped onion and minced garlic; cook until translucent and fragrant, about 5 minutes, stirring occasionally.
Add the ground lamb to the skillet. Break it apart with your spoon and cook until deeply browned, about 8-10 minutes, until it develops a rich aroma and a caramelized exterior.
Toss in diced carrots and frozen peas, stirring to combine. Cook for about 5 minutes until the vegetables soften slightly and the mixture becomes fragrant.
Pour in the broth and stir, letting the mixture simmer until it thickens slightly and becomes rich, about 10 minutes. Season with salt, pepper, and Worcestershire sauce for added depth.
Meanwhile, boil the peeled and chopped potatoes in salted water until tender, about 15-20 minutes. Drain well and mash with butter and milk until smooth and fluffy.
Preheat your oven to 200°C (390°F). Spread the savory meat filling evenly in your casserole dish.
Spoon the fluffy mashed potatoes over the filling, smoothing the top with the back of a spoon or spatula. For a crispy crust, brush the top with melted butter.
Bake uncovered for 25-30 minutes, until the top is golden and bubbling around the edges. Sprinkle with shredded cheddar cheese in the last 5 minutes for a cheesy crust.
Remove from oven and let rest for 5-10 minutes to allow the filling to set. Serve hot, enjoying the comforting layers and crispy topping.
