Heat the oven to 450°F and arrange a rack in the middle.
Place the carrots on a baking sheet, drizzle with olive oil, season with salt and pepper, and toss to evenly coat. Roast, shaking the pan occasionally, until a knife easily pierces the carrots, about 10 to 15 minutes. (Smaller carrots will cook in less time.) Remove from the oven and set aside while you prepare the herbed mustard-butter.
Melt the butter in a large frying pan over medium heat until foaming. Add the mustard and stir to combine. Add the roasted carrots and herbs and toss to coat the carrots in the butter mixture. Season with salt and pepper as needed.