Spinach is notable for its relatively high iron content in comparison with other vegetables. Many vegetables contain between 1 and 3 milligrams of iron per 100 gram but it contains 4 milligrams per 100 grams.
Try to use fresh and cook while still wet without adding any extra water; cover the pan and shake it from time to time. It cooks down tremendously. Fresh leaves should be crisp and dark green.
Beetroot leaves look like very similar to spinach and are often sold as such but it is actually a completely different plant, related to beetroot but grown for the leaves rather than the root. The leaves are larger and the stems coarser; remove these when washing. Otherwise, treat exactly like spinach.