Place your peeled sweet potatoes in a large pot, then fill the pot with cold water until the potatoes are completely submerged. Place the pot on the stove and bring to a boil over medium-high heat.
Boil the potatoes until they are fork-tender. Then drain and place them in a large mixing bowl, or return them to the pot off the heat.
Add the butter, then using a potato masher or an electric mixing, mash the potatoes until they are smooth and creamy. Add salt and white pepper to taste, followed by the shredded cheddar cheese. Stir until the cheese is melted, then cover and set aside until ready to use.
Brown the ground lamb with the onion and garlic in a large saucepan over medium heat. Once the lamb is completely cooked through drain off and discard any excess grease, and return the pan to the heat.
Add the remaining vegetables and cook, starting often until the cabbage is wilted and the carrots have begun to soften. Add the seasoning, ketchup, and Worcestershire sauce, stir, and continue to cook until fragrant.
Sprinkle the flour over the meat mixture and stir until everything is coated in the flour, then add the beef broth. Bring the mixture to a simmer and cook stirring often until the mixture has thickened, then remove the pan from the heat.
Preheat the oven to 375 degrees F.
While the oven is preheating, spoon the lamb and vegetable mixture into a 9 x 13 casserole dish. Spoon the sweet potato mixture over the top, and gently spread it out with the back of your spoon, until it is completely covering the meat mixture.
Bake for 25-30 minutes or until hot and bubbly. Remove the pan from the oven and serve immediately.
Enjoy!