Heat oil in a pan and fry the onion till lightly brown. Add the ginger garlic paste and fry till the raw smell from the ginger garlic paste has vanished.
Add the mutton pieces and and cook till all the water dries up.
Now add the red chili powder, coriander powder, turmeric powder and salt and mix well.
Add a cup of water and close the lid of the pan. Cook till the mutton is just about done. If necessary add in more water.
While your mutton is getting cooked, get your spinach ready.
Take a large bowl of ice water and set this aside.
In another large pan, take more water. Add salt to it and bring it to a boil.
When the water starts boiling, add the spinach leaves. Let it rest for about 15-20 seconds. Remove the leaves from the hot water and dip them in ice cold water. Remove from the cold water immediately.
Place these blanched spinach in a blender and make a coarse paste.
Add this spinach puree to the mutton curry and mix well. Cook on simmer till the mutton is fully cooked.
Add the cumin powder and garam masala powder. Give it a stir and add the coriander leaves for garnish. Remove from heat.
Serve hot with roti, paratha or rice.