01 - Preheat oven to 400°F. Prick each russet potato several times with a fork, then rub them thoroughly with 2 tablespoons of olive oil, ½ teaspoon of salt, and ¼ teaspoon of black pepper. Place the prepared potatoes directly on the oven rack.
02 - Bake the potatoes for 45–50 minutes, or until they are completely tender when pierced with a fork. Carefully remove from the oven and allow them to cool slightly until manageable.
03 - Once cooled, cut each potato in half lengthwise. Using a spoon, carefully scoop out most of the potato flesh, leaving a ¼-inch thick shell around the edges. Reserve the scooped potato flesh for the mashed potato topping.
04 - Lightly brush the exterior and interior of the potato skins with additional olive oil. Return the potato skins, cut side up, to the oven and bake for 8–10 minutes, or until they are crisp and slightly golden.
05 - While the potato skins crisp, mash the reserved potato flesh in a bowl. Incorporate 2 tablespoons of unsalted butter and ¼ cup of whole milk. Season with salt and pepper to taste, mashing until the mixture is smooth and creamy.
06 - Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the finely chopped small onion, diced carrot, and diced celery. Sauté for approximately 5 minutes, or until the vegetables have softened. Stir in the minced garlic and cook for another minute until fragrant.
07 - Add 12 ounces of ground beef or lamb to the skillet with the vegetables. Cook, breaking up the meat with a spoon, until it is thoroughly browned and no pink remains. Drain any excess fat.
08 - Stir in 1 tablespoon of tomato paste, 1 teaspoon of Worcestershire sauce, 1 teaspoon of fresh thyme leaves (or ½ teaspoon dried), and ½ cup of beef or vegetable broth. Bring the mixture to a simmer and cook for 5 minutes, allowing the sauce to thicken slightly.
09 - Add ½ cup of frozen peas to the skillet. Season the filling generously with salt and pepper to taste. Remove the skillet from the heat.
10 - Spoon a generous amount of the prepared shepherd's pie filling into each crispy potato skin. Top the filling with the mashed potato mixture, spreading it smoothly or piping it over the filling for a decorative finish. If using, sprinkle ½ cup of grated Irish cheddar cheese over the mashed potato topping.
11 - Return the assembled potato skins to the oven. Bake for an additional 10–12 minutes, or until the mashed potato topping is golden brown and the cheese (if used) is melted and bubbly. Serve hot immediately.