Heat oil in a pressure cooker or thick bottomed pan and add cumin seeds.After one minute or so when cumin seeds start spluttering, add chopped onion and saute for 4-5 minutes till onion become light brown in colour.
When onion are cooked add chopped tomatoes,turmeric, red chiili powder and cook till they are mashed .Now add 3-4 tablespoon of water and cook the masala for 5 minutes or till it stops sticking to bottom.
Now add potatoes/Aloo and shelled peas/Matar, cook for 5 more minutes and add powder and mix them with the help of a ladle. Add chopped mango pickle.
Add one cups of water and cook Aloo matar till two whistles if you are cooking in a pressure cooker or if you are cooking in normal pan cook it covered till potatoes are well cooked and tender generally that will take about 20-25 minutes on slow heat.
If I am serving Aloo matar with rice I prefer this texture and when I am planning to serve it with poori parantha I mash 7-8 cubes of potatoes with the help of ladle and cook for 2-3 more minutes to make a thicker gravy.