Heat oil in a skillet over medium heat; add onions. Cook, stirring occasionally, 7 minutes or until soft. Add cider, vinegar, and sugar. Stir until sugar dissolves and onions are evenly coated. Add sea salt and pepper; reduce heat to low. Simmer, stirring occasionally, 7 minutes or until liquid has reduced to a glaze. Add thyme, and stir. Sprinkle with additional sea salt, if desired, and serve warm.