Peel and cut potatoes into small cubes. Use fresh peas, if possible. If using frozen peas, soak them in water for 15 minutes before cooking.
Heat oil in a medium skillet, add onion and fry until the onions are soft and the edges are brown.
Add crushed garlic and ginger and stirring frequently fry these for two minutes. Then add chili powder, turmeric powder, coriander powder and salt, stir once and quickly add chopped tomatoes and 3-4 tablespoons of water.
Stirring constantly fry tomatoes on medium-low heat for 5 -8 minutes or until the tomatoes are reduced to pulp, mashing tomatoes with the back of the spoon. Add little water if the mixture sticks to the bottom of the skillet.
Put in potatoes, peas and 1 cup of water and stir to mix the ingredients. Bring the mixture to boil over high heat, and then reduce heat to low, cover and cook on low heat for 15 minutes or until the vegetables are tender and most of the water has evaporated. Add more water for thin gravy and adjust the seasoning with salt.
Just before serving sprinkle with garam masala, finely chopped coriander leaves and sliced green chillies.