1. Preheat oven to 425 degrees Fahrenheit. Line a large roasting pan with Gefen Parchment Paper.
2. Scrub carrots and slice onions in half from top to bottom.
3. Place carrots and onions (cut-side down) on the roasting pan. Drizzle with balsamic vinegar, olive oil and honey and sprinkle with salt.
4. Roast for 30 minutes, turning halfway through, so that the cut sides of the onion can brown.
5. Remove vegetables from the pan. Reserve any remaining liquid and set aside. Serve onions alongside the rib roast. The carrots can be served alongside the meat or as a separate dish.
6. Arrange carrots on a serving dish. Add mayonnaise to the reserved liquid and mix well, then drizzle over carrots.
7. Sprinkle with cilantro and pistachios before serving.