01 - Preheat oven to 400°F. Scrub russet potatoes thoroughly, pierce each several times with a fork, rub with olive oil, and season generously with salt. Arrange potatoes directly on oven rack and bake for 50-60 minutes until completely tender when pierced with a knife.
02 - While potatoes bake, heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add ground lamb, breaking apart with a wooden spoon, and cook until browned completely, about 6-8 minutes.
03 - Add diced onion, carrots, and minced garlic to the browned lamb. Cook, stirring occasionally, until vegetables soften and onions become translucent, approximately 5-7 minutes.
04 - Stir in tomato paste, Worcestershire sauce, dried thyme, and dried rosemary. Sprinkle flour or cornstarch over the mixture and cook constantly stirring for 1 minute to remove raw flour taste.
05 - Pour in stock gradually while stirring. Bring mixture to a gentle simmer and cook until sauce thickens nicely, about 5 minutes. Fold in frozen peas and season thoroughly with salt and pepper. Remove from heat.
06 - Once baked potatoes are cool enough to handle, cut a lengthwise slit in each. Gently fluff interior flesh with a fork, being careful not to damage the skin structure.
07 - Scoop out approximately one-third of the potato flesh from each and transfer to a mixing bowl. This will create space for the filling while providing material for the mashed topping.
08 - Fill each hollowed potato generously with the shepherd's pie filling, mounding slightly above the potato opening.
09 - Mash the reserved potato flesh with butter, milk or cream, and cheddar cheese until smooth and creamy. Season with salt and pepper to taste.
10 - Spoon or pipe the mashed potato mixture over each stuffed potato. Arrange on a baking sheet and broil for 3-5 minutes until topping develops golden-brown coloration.
11 - Remove from oven, sprinkle with freshly chopped parsley, and serve immediately while hot.