Preheat oven to 425ºF. Line a large rimmed baking sheet with foil; spread carrots on sheet. In a bowl, whisk oil and mustard. Pour over carrots and turn to coat. Sprinkle with salt, pepper and orange zest. Nestle orange slices and rosemary sprig among carrots; drizzle with orange juice. Tightly cover baking sheet with foil; roast until carrots are nearly tender, about 20 minutes.
Remove from oven; drizzle with maple syrup. Roast, uncovered, until lightly caramelized and tender, about 10 minutes longer. Season with more salt, if desired.