Heat a large skillet over medium high heat and add olive oil. Add onions, cooking for 2-3 minutes until soft, then add garlic. Add in beef, salt and pepper and stir, just cooking until browned, about 4-5 minutes. Add beef to a slow cooker along with milk, ketchup and mustard, stirring well. Add in cubed cheddar, mixing to coat. Cover and cook for 3-4 hours on low, stirring once every 30 minutes or so, especially on the edges.