01 - Preheat the oven to 400°F.
02 - Boil the sweet potatoes in salted water for 15–20 minutes, until fork-tender. Drain well.
03 - Mash the sweet potatoes with butter, milk, salt, and pepper until smooth. Set aside.
04 - Heat olive oil in a large skillet over medium heat. Add onion and garlic, sauté for 2–3 minutes.
05 - Add carrot and celery; cook for 4–5 minutes until softened.
06 - Add ground beef or lamb; cook until browned, breaking it up with a spoon. Drain excess fat if needed.
07 - Stir in tomato paste, Worcestershire sauce, thyme, and broth. Simmer for 8–10 minutes until thickened.
08 - Stir in peas and season with salt and pepper. Remove from heat.
09 - Spread the filling evenly in a 9-inch square baking dish. Spoon the mashed sweet potatoes over the filling, spreading to cover evenly.
10 - Bake for 20–25 minutes until the top is lightly golden and filling is bubbling.
11 - Let rest 5 minutes before serving.