01 - Preheat oven to 400°F. Scrub the russet potatoes thoroughly, poke several holes with a fork, rub with olive oil, and season with salt. Place directly on the oven rack and bake for 50 to 60 minutes until tender when pierced with a fork.
02 - While potatoes bake, heat olive oil in a large skillet over medium-high heat. Add ground lamb and cook until browned, breaking up with a spoon as it cooks, approximately 6 to 8 minutes.
03 - Add diced onion, carrots, and minced garlic to the skillet. Sauté until vegetables are softened, 5 to 7 minutes.
04 - Stir in tomato paste, Worcestershire sauce, dried thyme, and dried rosemary. Sprinkle flour or cornstarch over the mixture and cook for 1 minute, stirring constantly.
05 - Pour in the stock and bring to a simmer. Cook until the gravy thickens, about 5 minutes. Stir in frozen peas and season with salt and pepper to taste. Remove from heat.
06 - Once potatoes are baked, let cool slightly. Cut a lengthwise slit in each potato and gently fluff the inside with a fork. Scoop out approximately one-third of the flesh from each potato, reserving in a bowl for the topping.
07 - Spoon a generous amount of shepherd's pie filling into each prepared potato, packing lightly.
08 - Add butter, milk or cream, shredded cheese, salt, and pepper to the reserved potato flesh. Mash until smooth and creamy.
09 - Spoon or pipe the mashed potato mixture over the filled potatoes. Place on a baking sheet and broil for 3 to 5 minutes until the top is golden brown and lightly crisped.
10 - Sprinkle with freshly chopped parsley and serve immediately while hot.