Wash and peel the colocasia roots. Slice into about 1 cm sized chunks. The colocasia could be slippery, so be careful the knife doesn't slip while slicing.
Wash the cabbage leaves and the mustard greens and dry them gently with a kitchen towel.
Wash, dry and slice the green beans into 2 cm lengths.
Place the water in a pan and bring to boil on high heat.
Add colocasia slices and again bring to a boil on high heat.
Reduce the heat, cover and cook the colocasia for about 10 minutes, till it turns soft. I kept the lid of the pan half open so that the water would not boil over the edge. To check if the vegetable is soft, lift one piece out and try to smash it with the back of a spoon. If it gets mashed, it is soft enough.
Add the shredded cabbage and greens, stir them in and cook for about 5 minutes.
Add salt (add a little, taste and add a little more if required).
Continue to cook for 5-8 minutes, stirring and mashing the colocasia at the sides of the pan with a spoon. Hold the pan steady with one hand while doing this, so that it does not tilt/fall.
When the liquid in the pan turns a milky color and reduces and thickens, turn off the stove and transfer the Hinkejvu to a serving dish.
Hinkejvu is an accompaniment, in Nagaland, to one or more of their spicy meat curries, chili sauces, and rice. It makes a great side dish to a spicy South Indian kara kozhambu too, which is how I had it.