Fresh herbs, Worcestershire sauce and ground pepper carry the flavor in this quick-cooking shepherd's pie with a sweet potato topping. Ground beef adds a rich, savory flavor, but ground turkey or ground meat substitute would work well too.
Preheat oven to 425°F. Place sweet potatoes in a large saucepan; add water to cover by 1 inch. Bring to a boil over high heat. Reduce heat to medium; simmer, undisturbed, until fork-tender, 10 to 12 minutes. Drain the sweet potatoes and return to the pan.
Cook the sweet potatoes over low heat, stirring constantly, until all water has evaporated, about 3 minutes. Remove from heat and mash with a potato masher until mostly smooth. Stir in sour cream, butter and 1/4 teaspoon salt. Set aside.
Whisk cornstarch and broth in a small bowl; set aside.
Heat a large ovenproof skillet over medium-high heat. Add ground beef and cook, undisturbed, until browned on the bottom, about 4 minutes. Continue to cook, stirring and breaking up the meat with a wooden spoon, until no longer pink, about 3 minutes. Add carrots, onion and garlic; cook, stirring often, until the carrots and onion soften slightly, about 6 minutes. Add tomato paste and herbs; cook, stirring constantly, for 1 minute. Stir the cornstarch mixture and add to the beef mixture along with peas, Worcestershire, pepper and the remaining 1/4 teaspoon salt. Cook, stirring often, until the mixture thickens, 2 to 3 minutes. Remove from heat. Evenly spread the mashed sweet potato mixture over the beef mixture in the skillet. Using a spoon, decoratively swirl the top. Bake until the topping is hot, 15 to 20 minutes.