Blend the tomatoes and onion to make a thick puree.
Heat oil in a pot, add mustard seeds and cumin to this. Once the seeds splutter, you can put in the garlic, ginger, green chillies and curry leaves. Stir and let this fry for a minute or 2. It's now time to add all the powdered spices along with a splash of water. The water will help bring the temperature down and avoid the spices from burning.
In 2 minutes pour in the puree and cook this for 5-7 minutes stirring occasionally. Add the water, sugar and salt and continue to bring to a boil for another 10 minutes on low heat.
Notes
The humble tomato curry is ready to serve. Back home we always had this with rice and a side of fried fish or a fish salad or an omelette.