Fry ½ teaspoon of mustard seeds, as soon as they start popping, add sliced onion (½ medium sized)
When the onion is translucent/clear add your spices:
½ teaspoon red chilli powder
¼ teaspoon turmeric powder
½ teaspoon cumin powder
¼ teaspoon coriander powder
and a pinch of cinnamon
Allow the spices to fry in the oil for a minute.
Then add your tomatoes. These can be chopped fresh (3 large) or canned* (1-2 cans of whole/diced) which when cooked you can blend with a stick blender to puree.
I also add ½ teaspoon of ginger paste and the same of garlic paste (ground fresh is best tho!)
I allow the tomatoes to cook for 10-15minutes and then blend them into a sauce which I simmer for another 5-10minutes.
* Processed tomatoes have higher levels of lycopene, which is an antioxidant, in them.