01 - Preheat oven to 400°F. Scrub russet potatoes thoroughly, pierce several times with a fork, coat with olive oil, and season generously with salt. Place directly on oven rack and bake for 50–60 minutes until completely 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, breaking up with a wooden spoon, until browned and no longer pink, approximately 6–8 minutes.
03 - Add diced onion, carrots, and minced garlic to the browned lamb. Sauté until vegetables are softened and fragrant, about 5–7 minutes.
04 - Stir in tomato paste, Worcestershire sauce, dried thyme, and rosemary. Sprinkle flour or cornstarch evenly over the mixture and cook for 1 minute, stirring constantly to eliminate raw flour taste.
05 - Pour in 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, let cool until safe to handle. Cut a lengthwise slit in each potato and gently fluff the interior flesh with a fork.
07 - Scoop out approximately one-third of the potato flesh from each potato and transfer to a mixing bowl. This will become your mashed potato topping.
08 - Spoon a generous amount of shepherd's pie filling into each hollowed potato, packing lightly and mounding slightly above the potato surface.
09 - Add butter, milk or cream, cheddar cheese, salt, and pepper to the reserved potato flesh. Mash until completely smooth and creamy.
10 - Spoon or pipe the mashed potato mixture over the filling, creating a decorative pattern if desired. Place potatoes on a baking sheet and broil for 3–5 minutes until topping is golden brown and slightly crisped.
11 - Remove from oven, sprinkle with freshly chopped parsley, and serve immediately while hot.