Heat the olive oil in a large skillet over medium heat. Add the shredded cabbage and chopped onion, sautéing until the vegetables are tender and the cabbage is slightly caramelized, about 8 minutes.

Stir in the cooked brown rice, salt, and black pepper. Cook for an additional 5 minutes, stirring occasionally, until the rice is heated through and the flavors are well combined.

If desired, add apple cider vinegar for a tangy flavor, and cook for another 2 minutes. Serve warm as a nutritious side dish.