Make the masala
Take chopped onions, tomatoes, green chillies, ginger and garlic paste in a blender and blend to a fine paste.
Heat oil in a kadhai. Add cumin seeds. Allow them to sputter.
Now add in the masala paste. Be careful as it will splatter. Mix well. Now add turmeric powder and salt and let it cook on low heat.
I use a cover while cooking. Keep stirring to avoid the masala burning or sticking. Cook till the raw smell of the masala goes and you see oil separating from the masala.
Now add the cubed potatoes and green peas. Add a cup of water and simmer on low heat till the potatoes and peas are cooked.
Add coriander powder, chilly powder and garam masala powder.
Taste and adjust seasoning. You may add more water if you prefer a thin gravy.
Garnish with chopped coriander leaves.
Serve hot with chapatis or boiled rice.