Put 2 large onions, potatoes and tomatoes in a pressure cooker with 7 cups of water. After the first whistle, lower the flame and pressure cook for 10 mins.
Put off the gas and allow to cool. When cool, remove the onions, potatoes and tomatoes. Peel off the skin of the potatoes and tomatoes.
Grind to a paste in a mixer. If the mixture is too dry, use the water in the pressure cooker in which the onions, potatoes and tomatoes were cooked.
In a vessel heat 2 tsp of butter. In it saute finely cut small onions, when the onions turn glassy and soft, pour the ground mixture. Add the water in which onions, potatoes and tomatoes were cooked to get consistency required for the soup. Add the pepper powder, 4-5 tbsp of milk and salt to taste. Mix well, put vessel on fire, and stir continuously till soup boils. Put off the gas and serve hot.