Cut the pav in half.
Cook the potatoes in a pan of boiling water for 20 minutes, or until soft. Drain and allow to cool, then peel off the skins, return to the pan, and mash.
Make a small cross in the top of the tomatoes with a knife, then put them in a bowl of hot water and allow them to stand for about 5 minutes. Drain and allow to cool slightly, then peel off the skins. Set the tomato pulp aside.
Cook the peas in another pan of boiling water for 5-7 minutes, or until soft.
Heat a large Tawa or flat griddle and coat it with a thin film of butter by rotating the slab of butter all over the pan. Once Tawa is coated, remove the butter and set it aside.
Place the sliced pav on it and cook for about 3-4 minutes, or until slightly browned, then remove to a corner of the pan and flatten a little with a spatula.
Return the butter to the Tawa or griddle, concentrating half in the centre, where it is hottest, and when very hot, add the tomato pulp, mashed potatoes and other vegetables.
Add the ground spices and continue to mash with a spatula until extremely well blended. You may decide upon the consistency of the bhaji according to individual taste.
Serve hot, garnished with chilies, ginger, and coriander, accompanied with the pav and a small knob (pat) of butter, if desired.