Heat oil in a large pot or Dutch oven over medium heat. Add onions and cook, stirring, until the onions start to turn golden, about 8 minutes. Add cabbage and cook, stirring occasionally, until the cabbage wilts, about 5 minutes. Stir in broth, caraway seeds and sugar; bring to a simmer. Cover and cook over low heat until very tender, 15-20 minutes. Uncover and stir in vinegar. Increase heat to high, and cook, sirring occasionally, until most of the liquid has evaporated, 8 to 10 minutes. Season with salt and pepper.