Heat the oil in a large frying pan over medium-high heat. Once shimmering add the sliced red onion and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring occasionally, until tender, about 3 minutes. Add the 2 cloves chopped garlic and cook a couple minutes more.
Add the head of chopped cabbage and season again with 1 teaspoon salt and 1/4 teaspoon pepper. Lower heat to medium, and cook, stirring occasionally, until tender, about 20 minutes.
Add the 1/2 cup vinegar, 1/3 cup brown sugar and 1 teaspoon cumin seeds and lower heat to medium-low. Cook, covered, until the juices are syrupy and cabbage is soft, about 10 minutes more. Season with additional salt and pepper, if necessary and serve.