Heat oil in a large pot. Add cumin seeds. In a blender make puree of onion, tomatoes, garlic cloves and green chillies. Cook cumin for 2 minutes. Add chopped cilantro. Cook 2 minutes, add puree and cook covered for 5 minutes. Add a little water (4 tablespoons).
Add vegetable masala and coriander powder, and add 2 cups of water. Reduce heat to simmer and cook covered 10 mins. Add boiled potatoes and zucchini. Add garam masala and cook 10 mins. on slow heat (covered). Serve when ready to eat.