01 - Preheat the oven to 400°F (200°C).
02 - Place sweet potatoes in a large pot, cover with cold water, add a pinch of salt, bring to a boil, and simmer for 15 to 20 minutes until tender.
03 - Drain sweet potatoes and mash with butter, milk, salt, and pepper until smooth. Set aside.
04 - Heat olive oil in a large skillet over medium heat. Cook onion for 2 to 3 minutes until softened, then add garlic and carrots and cook for another 3 minutes.
05 - Add ground beef to the skillet, breaking it up with a spoon, and cook until browned, about 5 to 7 minutes.
06 - Stir in tomato paste, Worcestershire sauce, thyme, and rosemary, cooking for 2 minutes.
07 - Pour in beef broth, bring to a simmer, and cook for 5 minutes until slightly thickened.
08 - Stir frozen peas into the filling and season well with salt and pepper. Remove from heat.
09 - Spoon the beef mixture into a 2-quart baking dish and evenly spread the sweet potato mash over the top.
10 - Use a fork to create ridges on the sweet potato mash surface for crispiness.
11 - Bake for 20 to 25 minutes until the top is golden and the filling is bubbling.
12 - Allow to rest for 5 minutes before serving.