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.