In a large pot of water add sweet potatoes and bring to a boil over medium high heat. Boil for about 20 minutes or until fork tender. Drain the water. Mash with the butter and add salt and pepper to taste.
Preheat oven to 375 degrees. In a medium sized skillet add the ground beef and onion. Salt and pepper to taste. Cook and crumble the beef until no longer pink and onions are tender. Drain the grease. Add frozen vegetables, beef gravy and Worcestershire. Simmer for 4-5 minutes until the mixture starts to thicken.
Spoon the mashed sweet potatoes on top of the mixture. Bake in the oven for about 15 minutes until heated throughout. Broil the top of the potatoes for 2-3 minutes until it has browned.