Wash masoor dal and cook with ½ cup of water until dal is cooked but firm. This takes about 10-15 minutes.
Add coconut milk and continue cooking for another 3-5 minutes.
Meantime, heat oil in a pan, add mustard seeds, cumin, fenugreek seeds and let mustard seeds pop.
Throw in curry leaves, green chilies, onions and saute until onions are soft.
Add tomatoes, salt, turmeric and cook until tomatoes are soft, and mushy.
Transfer onion tomato mixture to cooked masoor dal, keeping aside a some mixture for garnish.
Check consistency of dal, add water if required, bring to boil and simmer for 5 minutes.
Garnish dhal curry with remaining onion tomato mixture.
Serve hot with rice along with few other non veg or veg curries.