Roasted Brussels sprouts get tossed in an apple-cider vinaigrette for a sweet, sticky coating. Reducing the apple cider until it's slightly thickened helps to concentrate its flavor and sweetness, giving the dressing an intense punch.
Preheat oven to 400°F.
Toss Brussels sprouts with 2 tablespoons oil and 1/4 teaspoon each salt and pepper on a large rimmed baking sheet. Roast until tender and lightly browned, 15 to 20 minutes. Add onion and roast until the onion is tender, 3 to 5 minutes. Add apple and roast until the apple is just tender, 3 to 5 minutes.
Meanwhile, bring apple cider to a boil in a small saucepan over medium-high heat. Boil until reduced to 1/4 cup, about 5 minutes. Transfer to a large bowl. Whisk in shallot, vinegar, mustard, maple syrup and the remaining 1/4 teaspoon each salt and pepper. Slowly whisk in the remaining 1 tablespoon oil.
Add walnuts and the Brussels sprouts mixture to the large bowl with the dressing. Toss to coat.