Cook plantain pieces either in enough water or steam them for 5 to 7 mins. Make sure the pieces do not turn mushy. If you boil them in an open pan you can keep an eye. Meanwhile you can proceed with the next step. Even if your pieces are parboiled or undercooked, its fine it gets cooked up later.
Heat a pan with oil, add mustard, cumin, curry leaves and green chili. Sauté till they begin to crackle.
Add ginger garlic paste and sauté quickly to get an aroma.
Add onions and sprinkle salt and fry till they turn golden. If you are using tomato, add and fry till the tomatoes turn mushy completely.
Add green peas if using, fry for 2 mins.
Add the boiled plantain pieces , add salt, turmeric, chili powder and garam masala.
fry on medium high for 3 to 4 mins or till fully cooked. Toss in between.