Heat a heavy, deep pan over medium flame. When the pan is hot, add the oil and then mustard seeds. Allow the mustard seeds to splutter.
Then add the cumin seeds. Reduce heat. Add chopped garlic and curry leaves. Sauté for a few seconds.
Add the chopped onion and sauté for about 1 minute. Add turmeric powder, chili powder, sugar, and salt and cook for another 30 seconds or so.
Add cubed tomatoes, increase the flame and sauté the tomatoes and remaining ingredients over high flame for 1-2 minutes.
Turn off heat and garnish with cilantro. Serve with Indian flatbread.