Mustard-Glazed Shallots and Carrots

Preheat oven to 450 degrees, with rack in middle. In a small bowl, whisk together oil, mustard, brown sugar, and vinegar. In a roasting pan, toss shallots and carrots with mustard mixture. Season with salt and pepper. Arrange vegetables in a single layer and roast until they are beginning to brown and are tender when pierced with a knife, about 40 minutes, stirring halfway through. Stir in butter and serve warm.