Trim any damaged leaves off the Brussels sprouts and trim the stem ends. Cut an X in the bottom of each sprout, then steam the sprouts in a vegetable steamer until crisp-tender.

In a medium mixing bowl combine the vinegar, maple syrup, shallots and mustard and whisk together. Let sit for 5 minutes. Whisk in the oil in a slow, steady stream until emulsified. Add salt and pepper, to taste.

When the Brussels sprouts are steamed to desired doneness, toss with the vinaigrette and walnuts and serve immediately.