Preheat the oven to 375°. In a large heatproof skillet, melt the butter in the olive oil over moderately high heat. Add the carrots, salt and pepper and cook until browned all over, turning occasionally, 3 to 4 minutes. Remove the skillet from the heat and add the vinegar and maple syrup. Shake the skillet to coat the carrots. Transfer to the oven and roast for 6 to 8 minutes, until the carrots are just tender but still crisp in the center. Transfer to a platter, top with the cashews and carrot greens and serve.