Drain the water from the peas and rinse well. Transfer to a pressure cooker.
Pressure cook in enough water with a pinch of salt till it is soft to the touch.
Add little water to the besan (chickpeas flour) and make into a paste without any lumps.
Pour the remaining water to the paste and mix well into a solution and keep aside.
Heat the oil in a deep pan. When hot, add onions. Sauté till they are translucent.
Add tomato puree and cook for a minute. Add red chilli powder and mix well. Cook for 2 more minutes.
Add cooked peas along with the water, salt and liquid besan (chickpeas flour) mixture and mix well.
Cook over low flame till the raw smell of the besan disappears.
Keep stirring in between as the besan might settle in the bottom and start to burn. This should take 2 to 3 mins.
At this stage you can see the thickening of the chaat with a saucy look. Take off the stove.
Peas chaat is ready.
Sprinkle chaat masala powder, mix well and keep the pan closed for 2 to 3 mins.