In a large non-stick frying pan, heat oil and fry turmeric, cumin, and curry powder for a few seconds. Add onion and fry until lightly browned. Add potatoes and fry until translucent. Add garlic, ginger, and chili and stir thoroughly with the onion/potato mixture for 5 minutes over low heat. Add vegetables except for the tomatoes and stir for a few more minutes. Now add the chopped tomatoes, water, salt and pepper; allow simmering 15-20 minutes before taking of the heat. Serve with steamed rice and roti (flat bread).