Heat a large skillet over medium-high heat. Cook and stir beef and sliced onion in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease.
Stir in sliced cabbage, tomato soup, water (use the empty soup can to measure), salt, and pepper. Cover and simmer until cabbage is tender, stirring occasionally, about 25 minutes. Stir in sour cream just before serving.