In the bowl of a food processor, puree the cauliflower and sweet potato together. Add the milk (dairy or soy) and egg. Puree until smooth. Season with salt and pepper. Mix again. If your food processor is too small, you may need to do this in batches. Transfer mixture to an oven proof casserole dish and reheat or re-cook in oven for 30 minutes before serving.
1 sweet potato, 1 egg, ¼ tsp salt, 1 pinch pepper, 2 tbsp soy milk