Preheat oven to 375 degrees F.

Place the potatoes on the oven rack and bake until a fork goes in easily, about 45 minutes.

In the meantime, place a large skillet over medium-high heat with the olive oil. Once hot, add in the ground sirloin and break it up with a wooden spoon. Cook until the beef has browned and is no longer pink. Drain and return to the skillet. Stir in the carrot, celery, garlic and scallions and cook until the vegetables begin to soften, about 5 to 8 minutes. Season with 3/4 teaspoon salt, black pepper, tomato paste and Worcestershire sauce. Cook about 1 minute longer and then stir in the flour. Stir in the beef stock, bay leaf, and thyme sprigs and bring to a simmer. Cook over low until the sauce has thickened and almost evaporated. Remove from the heat, stir in the peas, discard the bay leaf and thyme sprigs, and set off to the side.

Once the potatoes are done, let them cool slightly before cutting in half, lengthwise. Using a spoon, carefully scoop out most of the potato into a bowl, leaving some behind for a sturdy shell. Place the potato skins on a baking sheet and fill with the beef mixture.

To the bowl of potatoes, add the butter, buttermilk, the 1/2 teaspoon salt, 1/2 teaspoon black pepper and garlic powder. Mash until as smooth as possible. Transfer to a piping bag, fitted with a large star tip, and pipe out the mashed potatoes on top of the filling on each potato shell. Place under the broiler until golden brown on top. Remove from the oven and sprinkle the top with fresh thyme.