01 - Preheat oven to 400°F.
02 - Place sweet potato chunks in a large pot, cover with water, and bring to a boil. Cook for 15-20 minutes until fork-tender. Drain well.
03 - Mash 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, garlic, carrots, and celery. Sauté for 5-6 minutes until softened.
05 - Add ground beef to the skillet. Cook, breaking up the meat, until browned and no longer pink, about 6-8 minutes.
06 - Stir in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for 2 minutes.
07 - Pour in beef broth, bring to a simmer, and cook for 5 minutes until slightly thickened. Stir in frozen peas and cook for 2 more minutes.
08 - Transfer the beef mixture into a 9x13 inch baking dish. Spread evenly.
09 - Spoon the mashed sweet potato over the beef, spreading smoothly to cover.
10 - Bake for 20 minutes until the top is just starting to brown and filling is bubbling. Let cool for 10 minutes before serving.