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