01 - Preheat oven to 400°F for baking.
02 - Place sweet potatoes in a large pot of salted water. Bring to a boil, reduce heat, and simmer for 12-15 minutes until tender. Drain and return to the pot.
03 - Add butter, milk, salt, and pepper to sweet potatoes. Mash until smooth and creamy. Set aside.
04 - Heat olive oil in a large skillet over medium heat. Add onion and cook for 3 minutes until softened. Add garlic, carrots, and celery; cook for another 5 minutes.
05 - Add ground beef, breaking it up with a spoon. Cook until browned and no longer pink, about 5-7 minutes.
06 - Stir in tomato paste, Worcestershire sauce, thyme, and rosemary. Cook for 2 minutes to develop flavors.
07 - Pour in beef broth and bring to a simmer. Add peas, season with salt and pepper, and cook for 5 minutes until the mixture thickens slightly.
08 - Transfer beef mixture to an 8x8 inch baking dish. Spoon sweet potato mash over the beef, spreading evenly. Create ridges with a fork for a crispy top.
09 - Bake for 20-25 minutes until the top is golden and filling is bubbling.
10 - Let rest for 5 minutes before serving to allow the filling to set.