01 - Set oven to 400°F (200°C).
02 - Boil potatoes in salted water for 15 to 20 minutes until fork tender, then drain thoroughly.
03 - Mash potatoes with butter and milk until smooth. Stir in cheddar cheese if desired, and season with salt and pepper. Set aside.
04 - Heat olive oil in a large skillet over medium heat. Add onion, carrots, and celery; cook for 5 to 7 minutes until softened.
05 - Add minced garlic and cook for 1 minute. Add ground lamb, breaking apart with a spoon, and cook until browned and no longer pink, about 6 to 8 minutes.
06 - Stir in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for 2 minutes.
07 - Pour in stock, bring to a simmer, and cook for 10 minutes, stirring occasionally until slightly thickened.
08 - Stir in frozen peas and cook for an additional 2 minutes. Remove from heat.
09 - Transfer lamb mixture evenly to a 2-quart baking dish. Spread mashed potatoes over the top, covering completely, and create ridges with a fork.
10 - Bake in preheated oven for 25 to 30 minutes until the topping is golden and filling is bubbling.
11 - Allow the dish to rest for 10 minutes before serving.