Boil green peas in salted water till soft, drain, mash lightly and set aside. Grind ginger and garlic to a fine paste.
Heat oil in a pan and add three fourth quantity of onions. Sauté till light brown. Add green chillies and ginger-garlic paste. Stir-fry for half a minute.
Add half the quantity of tomatoes and cook on medium heat for three to four minutes, stirring continuously or till oil separates from the masala.
Add capsicum, mashed peas, cauliflower, potatoes and one and half cups of water. Bring it to a boil and simmer for ten minutes, pressing with back of the spoon a few times, till all the vegetables are completely mashed.
Add Pavbhaji Masala, salt and remaining tomatoes. Cook on medium heat for two minutes, stirring continuously. Heat half of the butter in a thick-bottomed pan or a tawa.
Slice pav horizontally into two and pan fry in butter for half a minute, pressing two or three times or till pav is crisp and light brown.
Garnish the bhaji with chopped coriander leaves, remaining butter and serve hot with pav accompanied with remaining chopped onion and lemon wedges.