Heat the oil in a large skillet over medium-high heat. Add the onions and garlic, stirring to coat in the oil. Cook for five minutes, stirring occasionally, until the onions are softened and golden.
Add the beef cubes to the skillet and cook for a few more minutes, until lightly browned.
Transfer the beef and onion mixture to a slow cooker. Add in the oregano, tomato paste, brown sugar, red wine, can of chopped tomatoes, olives, bay leaves and sweet potatoes. Stir to combine.
Cover and cook on low for four hours, stirring occasionally.
When done, serve the deliciously slow-cooked casserole.