Heat the oil and add the cumin seeds and allow them to crackle. When they crackle, add the onions and sauté till they are translucent. Add the garlic, ginger,green chilli paste and tomatoes and simmer till the tomatoes are cooked. Mash lightly using a potato masher and add 1 tbsp water. Add the green peas,potatoes, salt,chilli powder, garam masala and and cook for 3 to 4 minutes. Add 1 cup of water and allow it to come to a boil. Mash 2 to 3 pieces of potatoes in the pan to thicken the gravy. Simmer till the gravy thickens. Garnish with the coriander and serve hot with rotis or parathas.