Add the onion to the fat in the pot and cook, stirring occasionally, until deeply fragrant and lightly browned, about 1 minute. Add the cabbage and cook, stirring often, until the cabbage is golden brown in spots and wilted by half, about 7 minutes. Add the garlic and remaining miso mixture and cook, stirring occasionally, until the garlic is fragrant and the butter is melted, about 1 minute.