Begin by carmelizing the onions over medium-low heat in the tablespoon of butter until rich, dark brown (about 45 minutes)
For the vinaigrette: Spoon Dijon into the bottom of a jelly far. Add balsamic and maple syrup, salt, and pepper. Shake well. Add oil, and shake again. Taste and adjust for seasoning.
Mix greens with the other salad ingredients and toss with the vinaigrette.