Heat 2 T oil in a 30 cm / 12 inch, deep sided pan or dutch oven over a medium heat. Add potatoes and fry until medium brown, 5-8 minutes, stirring often. Remove with a slotted spoon and set aside.
Add the remaining 1/2 T oil to the pan and heat over a medium low heat. Add cumin seeds and fry for a few seconds until lightly browned. Add cabbage, green chilli and 1/4 cup water. Lower heat slightly and cook uncovered until cabbage is limp, 6-8 minutes, stirring often. Stir in salt, turmeric, sugar, red pepper, 2 T water and the potatoes.
Simmer, covered, until potatoes are tender, 18-20 minutes, adding peas during the last 2-3 minutes. Remove from heat and blend in garam masala. Let stand for a few minutes to help develop the flavours.