In a large pot, heat the oil on medium heat and brown the onions.
Once the onions are lightly browned, add the garlic and ginger paste and cook for a minute until fragrant.
Add the potatoes and toss in the oil and onions.
Add the turmeric powder, ground cumin, ground coriander and red cayenne powder.
Mix through into the potatoes.
Allow the potatoes to sauté for about 5 minutes.
Add enough water to cover the potatoes, cover and let potatoes cook, about 10-15 minutes.
Once the potatoes are just tender, making sure there isn't too much water, add the spinach leaves and cover to allow spinach to wilt.
The spinach leaves will release some water so you don't want too much water in the pan before adding the spinach, about a cup and half of water is acceptable.
Once the spinach has cooked down and beginning to get tender, uncover and evaporate some, of the water.
Add the green chile and the fenugreek leaves, if using.
Check for salt and adjust.
How dry you want the curry is up to you.
If you like the curry drier, then add the spinach leaves when there is barely any water left after the potatoes are done.