How to Make It
Preheat oven to 450°F. Line 2 large baking sheets with foil and brush lightly with oil. Toss Brussels sprouts, sweet potatoes and onion with 2 Tbsp. oil, red pepper flakes and salt. Arrange in a single layer on baking sheets. Roast in upper and lower thirds of oven, stirring occasionally, until golden brown and tender, 20 to 25 minutes.
In a large bowl, whisk together vinegar, mustard, honey and remaining 2 Tbsp. oil. Add vegetables, toss to coat and serve.