01 - Preheat oven to 400°F.
02 - Prick potatoes with a fork, rub with olive oil, salt, and pepper. Bake directly on oven rack for 45 to 50 minutes until tender. Let cool slightly.
03 - Cut potatoes in half lengthwise. Scoop out most of the flesh, leaving a 1/4-inch shell. Reserve scooped flesh for mashing.
04 - Brush potato skins with olive oil and return to oven, cut side up, for 8 to 10 minutes until crisp.
05 - Mash reserved potato flesh with butter, milk, salt, and pepper until smooth and creamy.
06 - Heat olive oil in a skillet over medium heat. Add onion, carrot, and celery; cook 5 minutes until softened. Stir in garlic, then add ground beef or lamb. Cook until browned, breaking up meat as it cooks.
07 - Stir in tomato paste, Worcestershire sauce, thyme, and broth. Simmer 5 minutes until thickened. Add peas and season with salt and pepper. Remove from heat.
08 - Fill each potato skin with a generous spoonful of filling. Top with mashed potatoes, smoothing or piping over the filling. Sprinkle with cheddar cheese if using.
09 - Return to oven and bake 10 to 12 minutes until tops are golden and cheese is melted. Serve hot.