Set the slow cooker to 110 °C / 230 °F (if it has low / medium / high settings consult your manual to determine the closest setting).
When the slow cooker is heated, add the olive oil, onions, garlic and black pepper. The aim is to gently cook with stirring for around 5 minutes, just to take away the raw aroma and taste but not to brown them.
Soon as the onions are cooked add the chopped tomatoes, oregano and brown sugar. Stir through to mix everything evenly. Place the lid on (make sure it's on properly to avoid drying out) and leave to cook - no further stirring is needed - for 4 hours.