Boil, peel and mash potatoes. If you want you can boil peas and cauliflower and mash them in too. Grind onions into a pulp. Heat butter in a pan and add ginger, garlic, onion paste, and cook. Add salt and turmeric. Put in chopped or pulped tomatoes. Add chilli powder and pav bhaji masala (you get that in a store) and fry. Add mashed veggies, some water and kasuri methi. Simmer until it looks as thick or runny as you prefer it. Turn off the heat and add chopped coriander leaves, and top with raw chopped onions and a dollop of butter. Serve with butter-toasted pav (dinner roll) and lime.
Turn it into a cheese pav bhaji by garnishing with shredded cheese. Or not.