Cook together sliced tomatoes, drumsticks, green chilli, 1/2 tsp turmeric powder and salt with 2.5 cups water. Bring it to a boil and cook on lowest flame, till tomatoes and drumstick becomes tender. Around 12 – 15 mins.
Grind together grated coconut, cumin, 2 small onion, chopped garlic, coriander powder and 1/4 tsp turmeric powder with 1/2 cup water for 3-4 mins. Add another 1/2 cup water and grind for 3-4 mins till it becomes a smooth paste.
Add 1/4 cup water to the coconut paste and mix well using a spatula. Add this to the cooked tomato and drumstick. Mix well. Cook on low flame for 7 – 10 mins. Do not let it boil.
Heat oil in a small pan and crackle the mustard seeds. Add sliced small onion, dry red chilli and curry leaves. Fry it till the onion becomes golden brown. Add a pinch of turmeric and chilli powder. Mix well and add to the curry. Let the curry rest for 15 minutes and serve with rice.