Heat oil and add cumin seeds, then the onion paste and saute till brown and fat separates.
Add the mixed vegetables and potatoes and sauté first over high heat, then over low, till cooked through.
Add tomatoes and puree, green chillies, turmeric, salt, pav bhaji masala and chilli powder and continue cooking and mashing vegetables while stirring, till fat separates.
Heat some butter, slit the bread horizontally and fry in the butter till brown. Serve with the vegetables.
Vegetables cooked and mashed together, Mumbai style.