Heat the oil in a pot over medium heat. Add the shallot, cabbage, and stems from the Swiss chard. Sprinkle lightly with sea salt and pepper. Cover and cook for about 10 minutes, stirring occasionally until the vegetables are barely tender. Add the Swiss chard leaves and the cider vinegar, then stir to combine. Cover and continue cooking for about 5 minutes, stirring occasionally until the vegetables are tender to your preference. Taste for seasonings.