In a medium soup pot, heat the olive oil until shimmering. Add the onions, cover and cook them over moderately high heat, stirring occasionally, until they are softened, 3 to 4 minutes. Uncover and cook until the onions are lightly browned, about 5 minutes longer. Add the brown sugar and cook for 1 minute. Add the tomatoes and their juices, the beef stock and lemon juice, season lightly with salt and pepper and bring to a boil. Simmer the soup over moderate heat for 10 minutes.