I understand that amaranth greens will be available in local markets in the next few weeks. I've never had them, so I looked into what they are, how to prepare them, etc.
The food is widely used in Indian cooking, and goes by several names (cheera, chaulai, etc). Both the seeds and leaves are used. The leaves may be steamed, mashed, fried, etc. Treat them like spinach, this site says.
Sailaja Gudivada in her popular blog Sailu’s Kitchen uses the leaves to make an interesting snack called thotakura undalu, which are crispy balls tempered with sesame seeds, curry leaves, chillies, hing and mustard seeds; and a mustard flavoured, tangy stew called thotakura ava pulusu.
She says, “Amaranth leaves are much loved in our home and take the form of stir fry, dal and stew in our daily meals. In recent past, I have used these nutritious greens along with chick pea flour (besan) to prepare fritters. They are easy to make, work great as appetizers for parties and also make for a wonderful tea time snack.”