01 - Preheat oven to 400°F. Scrub the russet potatoes, poke several times with a fork, rub with olive oil, and sprinkle with salt. Place directly on the oven rack and bake for 50-60 minutes, or until tender.
02 - While potatoes bake, heat olive oil in a large skillet over medium-high heat. Add the ground lamb and cook until browned, breaking it up as it cooks.
03 - Add onion, carrots, and garlic to the skillet. Sauté until vegetables are soft, 5-7 minutes.
04 - Stir in tomato paste, Worcestershire sauce, thyme, and rosemary. Sprinkle with flour or cornstarch and cook for 1 minute.
05 - Pour in the stock and simmer until thickened, about 5 minutes. Stir in the peas. Season with salt and pepper.
06 - Once potatoes are baked, let cool slightly. Cut a slit in each potato and gently fluff the inside with a fork.
07 - Scoop or press some potato flesh from each potato, reserving in a bowl for the topping.
08 - Fill each potato with a generous amount of shepherd's pie filling.
09 - Mix the reserved potato flesh with butter, milk or cream, cheese (if using), salt, and pepper; mash until smooth.
10 - Spoon or pipe the mashed potato topping over the filled potatoes. Place on a baking sheet and broil 3-5 minutes until golden.
11 - Sprinkle with fresh parsley and serve hot.