Wash spinach leaves in a large vessel and drain water in a colander.
In a pan, fry garlic on a medium low flame for 10 sec. Then add whole spices (cumin, fenugreek seeds, mustard seeds). Let them pop and then add powdered spices, coriander and turmeric.
Finally, add wet spices, green chili and ginger paste. Fry well so the flavors are released.
Add chopped onion and salt. Fry till onions are soft, around 3-4 minutes.
Now, add tomatoes add dry fenugreek leaves. Cover and simmer for 5 minutes on medium low flame till tomato soften and color darkens.
While gravy is simmering, chop spinach leaves,. Add potatoes and spinach leaves to the gravy too, cover and cook for 15 minutes.
Add butter and fry on high for 2 minutes untill oil release.
Garnish with green chilles and serve.