Watercress is a nutritionally sound green leafy staple food as it has a calcium to phosphrous ration of 2:1, it also has 2% protein and 0% fat.
The green mustardy, slightly spicy tasting leaves are also sources of Folate, Pantothenic Acid and Copper, Vitamin A, Vitamin C, Vitamin E (Alpha Tocopherol), Vitamin K, Thiamin, Riboflavin, Vitamin B6, Calcium, Magnesium, Phosphorus, Potassium and Manganese.
Both our Iggy and uromastyx love watercress, and will eat it from our hands.
For smaller lizards separate the leaves from the stalks and give just the leaves but if you have a uromastyx they do generally like stalky bits too, just break them into manageable bits. Our big iguana has the stalky bits left attached to the laves but not in big clumps as he can be a bit of pig at feeding times and gobble too much at once.
Watercress is not easy to grow in most peoples back gardens as it requires large quantities of semi-alkaline water but is readily available in most supermarkets but at over £1 per bag, so my alternative is to grow american landcress, it does have a slighttely higher ratio of phospherous than watercress it is still fine to feed to herbivorous lizards.
As with all foods for lizards never limit your reptiles to one type of food, always alternate between a variety of foods and never consistently offer one type of food.