How to avoid a watery mashed potato-cauliflower topping.
Once the cauliflower and potatoes are done boiling, drain them thoroughly in a colander. Let the potatoes sit in the colander to dry out while you work on the cauliflower.
Transfer the cauliflower to a fine-mesh sieve, and place a bowl on top of the boiled cauliflower. Push down and squeeze out as much water as you can. There will be a lot more water than you think! Or wrap the cauliflower in a clean dish towel and tie it up. Then squeeze it into a ball and wring out all of the excess water.