01 - Preheat the oven to 400°F.
02 - Place sweet potatoes in a large pot, cover with cold water, and add a pinch of salt. Bring to a boil and cook for 15-20 minutes until 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, carrots, and celery; sauté for 5-7 minutes until softened. Stir in garlic and cook 1 minute more.
05 - Add ground beef and cook, breaking it up with a spoon, until browned and cooked through, about 8 minutes. Drain excess fat if needed.
06 - Stir in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for 2 minutes.
07 - Pour in beef broth and bring to a simmer. Cook for 10 minutes, stirring occasionally, until slightly thickened. Stir in frozen peas and cook for 2 more minutes. Remove from heat.
08 - Transfer beef filling to a 9x13 inch baking dish. Spread sweet potato mash evenly over the top.
09 - Use a fork to create ridges on the surface for extra crispness. Bake for 20-25 minutes, or until the top is golden and filling is bubbling.
10 - Let rest 10 minutes before serving.