Bake the Potatoes. Preheat the oven to 400°F. Wash, dry, and pierce the potatoes with a fork. Brush with whisked egg white or olive oil and coat with salt, pepper, rosemary, and thyme. Bake for 55–65 minutes, or until tender; very large potatoes may need closer to 70 minutes. Cool for 15 minutes, then reduce the oven to 350°F.
Make the Mashed Potatoes. Cut off the top of each potato and scoop the pulp into a bowl, leaving about ¼ inch inside the skins. Mash with butter, cream or milk, garlic, Parmesan, salt, and pepper until creamy but thick enough to pipe.
Cook the Meat. Brown the ground meat over medium-high heat. When partially cooked, add the onion and continue cooking until the meat is cooked through and the onion is tender. Add the garlic and cook for 30–60 seconds.
Make the Filling. Sprinkle the flour over the meat and cook, stirring, for 2 minutes. Stir in the Worcestershire sauce and beef broth, then add the corn, peas, thyme, salt, and pepper. Simmer uncovered for 3–5 minutes, or until the gravy lightly coats the filling.
Assemble. Fill each potato shell about ⅔ full with the meat mixture. Pipe or spoon the mashed potatoes over the filling and top with shredded cheese.
Bake and Serve. Bake at 350°F for about 15 minutes, or until hot and the cheese has melted. Broil for 1–3 minutes until lightly golden. Serve hot.