Rinse the dal 3-4 times. Add 2 cups water, turmeric powder and salt to dal. Bring it to boil. Reduce flame to lowest and cook till it's done, around 12 - 15 mins.
Mash the cooked dal slightly with the back of the spoon. Add coconut milk gradually and mix well. Continue to cook on low flame for 6-8 mins.
Meanwhile in a small frying pan, heat the oil and add mustard, cumin and fennel seeds. Add chopped onion, crushed garlic, green chilli and chilli flakes. Cook till onion browns. Add this to the dal and mix well. Remove from fire.
Notes
The consistency of dal is medium thick. You can adjust it to suit your requirements.Keep in mind, that dal tends to thicken as it rests. Adjust the consistency accordingly. You can add curry leaves or chopped coriander leaves to the dal.