In a large, hot skillet over medium heat, cook the onions in oil and butter for 15 minutes, or until golden brown. Add a little more oil or butter as needed. Stir in the maple syrup, wine vinegar, and thyme. Cook for 10 minutes over medium-low heat, until the liquid has thickened and has a jam-like consistency. Remove the thyme sprig, then season with salt and pepper to taste.
