Prep the vegetables and boil it. Once it cools down, mash it well. In fact, you can make a puree of it or leave some chunks in it.
Heat a nonstick pan with butter, saute the finely chopped onions along with green chilies, garlic, saute well till the onion is nicely done.
Add the tomato puree, spice powders, and the mashed vegetables. Cook on high for the whole mixture gets cooked well.
Remember we are going to spread this on the pav, so the mixture should be a thick and even sauce.
Garnish with finely chopped coriander leaves.