Cook the potatoes until tender (do not overcook). Peel and set aside. In the container of a blender or food processor, puree the onions, garlic and ginger.
In a large heavy bottomed skillet over medium heat, warm the oil. Add the cumin, when the cumin darkens (1 to 2 seconds) add the purred onion mixture and sauté until browned (about 12 minutes).
Add the salt, cayenne, turmeric and tomatoes and cook until they soften (about 5 minutes). Add the peas and water, reduce heat to low, cover and cook for 5 minutes.
Halve the cooked potatoes and add them to the skillet and cook covered another 5 minutes. Mix in the garam masala just before serving
