You need: 1 big onion, finely chopped; 1 lb tomatoes, finely chopped; 1 bunch cilantro; chopped; salt to taste; oil; 2-3 curry leaves, 1/2 tsp mustard seeds
How to: Heat some oil. Add onions, cook for 4-5 minutes. Add the cilantro, stir, cook for another 2 minutes. Add the tomatoes and salt, stir well, and cook till it forms a thick paste. If you want it spicy, add some red chili powder at this point. Add a little water and cook some more, letting it reduce again to chutney consistency.
You can either blend it in a food processor once it cools, or leave it chunky.
Final step: heat 1 tsp oil,add mustard seeds and curry leaves. When it sputters, add to the chutney and mix in.