Preheat your oven to 400°F.
In a medium saucepan, peel and dice the sweet potato. Boil in lightly salted water until tender, about 10-12 minutes, then drain and mash with a pinch of salt.
Meanwhile, heat a non-stick skillet over medium heat. Add the lean ground beef and cook until browned, breaking it up as it cooks.
Add the chopped onion, carrots, and minced garlic to the beef. Sauté until the vegetables soften, about 4-5 minutes.
Stir in the tomato paste, green peas, beef broth, rosemary, thyme, salt, and pepper. Let the mixture simmer for 3-4 minutes to meld the flavors.
Transfer the beef and vegetable mixture into an ovenproof dish. Spread the mashed sweet potato evenly on top.
Bake in the preheated oven for 10-12 minutes until the topping is lightly browned and the filling is bubbly.
Remove from the oven, allow to cool slightly, and serve warm.