Meanwhile, combine the apple cider and cider vinegar in a small saucepan. Bring to simmer over medium-high heat and continue cooking until reduced to about ¼ cup, about 6 minutes. Remove from the heat and whisk in the Dijon mustard and ½ teaspoon salt. Measure out ½ cup of olive oil. Slowly, a few drops at a time, add the oil to the cider mixture, whisking constantly until it incorporates easily. Stream in the remaining olive oil while whisking until fully combined. The vinaigrette can be made up to 4 days in advance.