How to Make It
Preheat the oven to 425°. In a bowl, mix 6 tablespoons of the butter with 2 tablespoons of the mustard and the coriander. Season with salt and pepper.
On a large rimmed baking sheet, combine the carrots and fennel. On another large rimmed baking sheet, arrange the beets. Dollop the butter over the vegetables on each baking sheet and season with salt and pepper. Toss and rub to evenly coat. Roast for about 40 minutes, stirring occasionally and rotating the sheets halfway through, until the vegetables are tender and caramelized.
Meanwhile, in a small bowl, mix the remaining 2 tablespoons of butter and 1 tablespoon of mustard. Dollop the butter over the warm vegetables and toss to evenly coat. Transfer the vegetables to a serving platter and garnish with the dill.