Melt butter in a large saucepan over medium heat. Add onion, and cook stirring occasionally, for 10 to 15 minutes, until soft and golden brown. Add cabbage and saute, stirring occasionally, until tender about 25 minutes. Add vinegar, sugar, and cumin. MIx well, lower the heat to medium-low, and simmer until the juices are syrupy and the cabbage appears shiny, about 20 minutes. The cabbage should be tender but not mushy.