There are two reasons cauliflower can get watery. First, it’s overcooked. When it’s cooked too long, it releases water. You’re aiming for tender-crisp. In other words, not quite as fully cooked as you would like if you were just eating it plain. You’ll have to watch carefully when you put the whole dish in the oven as well. Start at the lower end of the cook time range and add extra time if needed. The second reason cauliflower might get watery is that it’s not fully cool when you add the cheese sauce. As it’s cooling, it releases steam. Cheese sauce will trap it in the pan, leaving the water nowhere to go but the pan. When you’re letting it cool after steaming, spread it out in a single layer on a towel-covered baking sheet if you want to be extra careful.