01 - Preheat oven to 400°F. Scrub russet potatoes thoroughly and pierce several times with a fork. Rub each potato with olive oil and season generously with salt. Place directly on oven rack and bake for 50–60 minutes until tender when pierced with a knife.
02 - While potatoes bake, heat olive oil in a large skillet over medium-high heat. Add ground lamb and cook until browned, breaking apart with a wooden spoon as it cooks, approximately 6–8 minutes.
03 - Add diced onion, carrots, and minced garlic to the browned lamb. Sauté until vegetables are softened and onions are translucent, 5–7 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 to eliminate raw flour taste.
05 - Pour in beef or vegetable stock and bring to a gentle simmer. Cook until sauce thickens nicely, about 5 minutes. Stir in frozen peas and season with salt and pepper to taste. Remove from heat.
06 - Once potatoes are baked and cool enough to handle, cut a lengthwise slit in each potato. Gently fluff the interior flesh with a fork, being careful not to tear the skins.
07 - Scoop out approximately one-third of the potato flesh from each potato, transferring to a mixing bowl. Reserve this flesh for the topping while keeping potato skins intact.
08 - Spoon a generous portion of shepherd's pie filling into each hollowed potato, packing lightly and mounding slightly above the potato opening.
09 - To the reserved potato flesh, add unsalted butter, milk or cream, and cheddar cheese if using. Mash until smooth and creamy. Season with salt and pepper to taste.
10 - Spoon or pipe the mashed potato mixture over the filled potatoes, creating peaks that will brown nicely. Place on a baking sheet and broil for 3–5 minutes until golden and crispy on top.
11 - Remove from oven and sprinkle with fresh chopped parsley. Serve immediately while hot and crispy.