For the meatballs, mix the mince, oatmeal, milk, mustard powder, oregano and garlic powder in a large bowl and season well with salt and pepper. Mould with your hands into 8 large balls.
Heat the oil in a large lidded skillet or frying pan. Add the potato, onion and carrot and fry for about 10 minutes, or until brown on all sides. Push to one side of the skillet and add the meatballs. Fry until browned all over, then gently push around the pan until they are evenly distributed amongst the vegetables. Add around 100ml/3½fl oz water, cover and leave to simmer for 15–20 minutes, or until the meatballs are cooked through and the vegetables are tender.
For the sauce, place all of the sauce ingredients, except the cheddar, in the skillet, stir and season with plenty of salt and pepper. Cook for 15–20 minutes, or until the sauce is reduced and thick. Serve with the Cowboy beans.