01 - Preheat oven to 400°F. Scrub the russet potatoes thoroughly, 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 until tender throughout.
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 with a spoon as it cooks, approximately 6-8 minutes.
03 - Add diced onion, carrots, and minced garlic to the skillet with the browned lamb. Sauté until vegetables are softened, 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 remove raw flour taste.
05 - Pour in the beef or vegetable stock and bring to a simmer. Cook until the mixture thickens nicely, about 5 minutes. Stir in frozen peas and season with salt and pepper to taste. Remove from heat.
06 - Once baked potatoes are cool enough to handle, cut a slit in the top of each potato and gently fluff the interior with a fork, being careful not to tear the skins.
07 - Scoop out approximately one-third of the flesh from each potato, reserving in a mixing bowl. Add butter, milk or cream, cheddar cheese if using, salt, and pepper. Mash until smooth and creamy.
08 - Fill each potato cavity generously with the shepherd's pie filling, packing lightly but not overflowing.
09 - Spoon or pipe the mashed potato mixture over the filling of each stuffed potato. Place on a baking sheet and broil for 3-5 minutes until the topping is golden brown and slightly crisp.
10 - Remove from oven, sprinkle with freshly chopped parsley, and serve immediately while hot.