01 - Preheat oven to 400°F.
02 - Place sweet potatoes in a large pot of salted water. Bring to a boil, reduce heat, and simmer for 15-20 minutes until tender. Drain well.
03 - Mash sweet potatoes with butter and milk until smooth. Season with salt and pepper. Set aside.
04 - Heat olive oil in a large skillet over medium heat. Add onion, garlic, carrots, and celery. Sauté for 5-7 minutes until softened.
05 - Add ground beef and cook, breaking up with a spoon, until browned. Drain excess fat if needed.
06 - Stir in tomato paste, thyme, rosemary, beef stock, and Worcestershire sauce. Simmer for 10 minutes, stirring occasionally, until mixture thickens slightly.
07 - Stir in frozen peas and cook for 2 minutes. Season with salt and pepper.
08 - Transfer beef filling to a 2-quart baking dish. Spread sweet potato mash evenly over the beef mixture. Use a fork to create ridges for a decorative finish.
09 - Bake in preheated oven for 25 minutes, or until topping is lightly browned and filling is bubbling.
10 - Let rest for 5 minutes before serving.