Preheat oven to 425 degrees. In a medium-sized skillet, heat butter over medium heat. Add onions and garlic to pot and cook until onions are translucent, about 5-6 minutes.
Meanwhile, cook sausage in another skillet over medium-high heat for about 8-10 minutes, or until cooked through and no longer pink. Drain away any excess fat.
Peel sweet potatoes, then cut into about ½-inch cubes. Toss with olive oil, salt, and pepper. Stir in onions and sausage.
Spread sweet potatoes in a large cast iron or oven-safe skillet. Roast the sweet potatoes in a single layer for 25-30 minutes, stirring occasionally, until they are soft and golden brown.