If you want to add cooked spinach to a dish, say, a dip, or crêpes, or a tart, some preparation is required. It’s very easy and takes no time at all, so I highly recommend doing this. It’s a way better alternative to using canned or frozen spinach*.
There are different ways to prepare spinach, but this is the method I use.
Place the spinach leaves inside a steamer over a pot of water. Steam the spinach for about 5 minutes. The giant pile of spinach will shrink to a very small amount, which just shows how much water there is in vegetables. See mushroom prep.
Turn the steamer over and dump the spinach on a clean dish towel. When it’s cool enough to work with, wrap up the spinach, squeeze, and gradually get all of the remaining water out. Then you’re left with a tight ball of spinach.
Simply chop up this spinach ball, or leave as is if you don’t want chopped spinach. But now you can use it in recipes without worrying that it’s going to produce any more liquid.
* Even with canned and frozen spinach you have to remove as much water as possible before using it in recipes, so you might as well start off with fresh spinach leaves.