Heat the oven to 500°F with a rimmed baking sheet inside. In a large bowl, whisk both mustards, the oil and lemon zest and juice, then season with honey and salt. Add the cauliflower and toss. Quickly remove the hot baking sheet from the oven and distribute the cauliflower in an even layer on it, scraping the bowl. Roast until well browned and tender, about 20 minutes, stirring once halfway through. Serve sprinkled with dill.