Set cauliflower and carrots in a steamer basket in a large pot, sprinkle with salt and steam for 20 minutes until soft.
Meanwhile, heat butter in a skillet over medium heat, add onions and cook until caramelized, about 25 minutes. Make sure to regulate the heat so that the butter remains hot but does not burn.
Add your minced garlic, sprinkle with salt and cook for one more minute.
Place steamed cauliflower and carrots in your food processor fitted with the large chopping blade. Pour the butter with caramelized onions and garlic over the carrots and cauliflower. Add one teaspoon of salt, a sprinkling of pepper and rosemary. Process until creamy. Serve hot.
What I love about this dish is that it can be prepared a few days before the holiday (which means I can get a head start on the holiday cooking). It will keep in the refrigerator up to four days in advance. When it's time to serve, I can just reheat the purée over a low heat on top of the stove (this also makes it a perfect side dish to bring to a holiday party along with gluten free stuffing!).