01 - Preheat your oven to 400°F. Thoroughly prick the scrubbed russet potatoes with a fork. Rub them evenly with 2 tablespoons of olive oil, ½ teaspoon of salt, and ¼ teaspoon of black pepper. Place the seasoned potatoes directly on the oven rack and bake for 45 to 50 minutes, or until they are tender when pierced. Allow them to cool slightly for easier handling.
02 - Carefully slice each cooled potato in half lengthwise. Using a spoon, scoop out most of the soft potato flesh, ensuring a sturdy ¼-inch (0.5 cm) thick shell remains. Set aside the scooped potato flesh; it will be used for the mashed topping. Lightly brush the exterior of the potato skins with a little olive oil. Return the potato skins, cut side up, to the oven and bake for an additional 8 to 10 minutes until they become crisp and golden.
03 - While the potato skins are crisping, prepare the mashed topping. In a medium bowl, combine the reserved potato flesh with 2 tablespoons of unsalted butter and ¼ cup of whole milk. Mash until smooth and creamy, then season with salt and pepper to your desired taste.
04 - Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the finely chopped onion, diced carrot, and diced celery to the skillet and sauté for about 5 minutes, until they begin to soften. Stir in the minced garlic and cook for another minute until fragrant. Add the 12 ounces of ground beef or lamb, breaking it up with a spoon as it cooks, until thoroughly browned. Drain any excess fat if necessary.
05 - Stir in the 1 tablespoon of tomato paste, 1 teaspoon of Worcestershire sauce, and 1 teaspoon of fresh thyme leaves (or ½ teaspoon dried). Pour in ½ cup of beef or vegetable broth and bring the mixture to a simmer. Allow it to cook for about 5 minutes, or until the sauce has slightly thickened. Stir in the ½ cup of frozen peas and season the filling generously with salt and pepper. Remove the skillet from the heat.
06 - Spoon a generous amount of the prepared shepherds pie filling into each crispy potato skin. Carefully top the filling with the mashed potato mixture, using a spoon to smooth it over or a piping bag for a more decorative finish. If using, sprinkle ½ cup of grated Irish cheddar cheese evenly over the mashed potato topping.
07 - Return the assembled potato skins to the oven. Bake for 10 to 12 minutes, or until the mashed potato tops are golden brown and the cheese (if used) is melted and bubbly. Serve these festive St. Patrick's Day Shepherd's Pie Potato Skins hot.