01 - Set the oven to 400°F to preheat.
02 - Place sweet potatoes in a large pot of salted water. Bring to a boil, reduce heat, and simmer for 15 to 20 minutes until tender. Drain thoroughly.
03 - Mash the drained sweet potatoes with butter and milk. Season with salt and black pepper. Set aside.
04 - Heat olive oil in a large skillet over medium heat. Add onion, garlic, carrots, and celery; cook for 5 to 7 minutes until softened.
05 - Add ground beef to the skillet, breaking it up as it cooks until fully browned with no pink remaining.
06 - Stir in tomato paste, Worcestershire sauce, dried thyme, dried rosemary, salt, and pepper. Cook for 2 minutes to combine flavors.
07 - Pour in beef stock, bring mixture to a simmer, and cook uncovered for 10 minutes until thickened. Add frozen peas and simmer for an additional 2 minutes.
08 - Transfer beef mixture into a 9x13 inch baking dish. Evenly spread the sweet potato mash on top.
09 - Bake in the preheated oven for 20 minutes until the topping is lightly golden and the filling bubbles.
10 - Allow to rest for 5 minutes before serving hot.