Steam or boil sweet potatoes until tender. Once cooked, mash with butter and ¼ teaspoon each salt and black pepper until smooth. Set aside.
Meanwhile, add the beef, onions and vegetables to a large skillet. Cook over medium heat, stirring frequently to crumble the beef, until the meat is cooked through and the vegetables are softened, about 10-15 minutes.
Reduce the heat to medium low and stir in the garlic for 1 minute follow by the remaining ¾ teaspoon salt, ¼ teaspoon of pepper and seasonings.
To a 3 quart or similar size casserole dish, add the meat mixture and then top with the mashed sweet potatoes.
Bake at 375 °F for 15 minutes, covering for the first 10 minutes and then removing the cover for the last 5.
Serve.
Notes
Make AheadAssemble the entire casserole, following the directions through up until baking but instead of baking, cover and refrigerate for up to 3 days. You can also freeze for up to 3 months.**If freezing, refrigerate for a few hours first before moving to the freezer so that your dish doesn't break from the temperature shock of going from hot to freezing.Bake at 375 F for 35-45 minutes, covered for the first 30 minutes and then uncovered for the rest.LeftoversStore leftovers in an airtight container in the refrigerator for up to 4 days or freeze for up to 3 months.Reheat in the microwave or in the oven at 375 F for 15-20 minutes (for individual portions) or 30-35 minutes (for the full casserole) until heated through.