In a small Saucepan, combine the cider and vinegar and bring to a boil. Continue to boil until the liquid is reduced to a little more than ½ cup; this will take 10 to 12 minutes. (Put ½ cup water in the pan before you start so you can eyeball the quantity you’re aiming for.) Remove the pan from the heat, and whisk in the Dijon mustard and olive oil. Taste and season with salt and pepper. Store covered in the refrigerator for up to 1 week.