Preheat the oven to 375 degrees. In a large bowl, combine the ground beef, onion, carrots, zucchini, bell pepper, egg, breadcrumbs, garlic, salt, and pepper. Mix until evenly blended.
Shape the mixture into a loaf and place it in a loaf pan. Bake for 45 minutes until the internal temperature reaches 160 degrees.
While the meatloaf bakes, place the sweet potatoes in a pot and cover with water. Bring to a boil and cook for 15 minutes until tender.
Drain the sweet potatoes and return them to the pot. Add the butter and milk, then mash until smooth.
Remove the meatloaf from the oven and let it rest for 5 minutes. Slice and serve with the mashed sweet potatoes. Sprinkle with parsley if using.