Place the lentils into a large bowl, fill it with water and gently massage them. Discard the foamy water and repeat until the water remains clear after massaging the lentils. Drain and set aside.
In a pot, heat 2 tbsp cooking oil until medium. Add the sliced onions (1/2 of a large onion) and fry until they turn translucent.
Add sliced garlic (3 cloves), sliced curry leaves (10-12 leaves), 1 tsp yellow mustard seeds and 1/4 tsp turmeric powder.
Once mustard seeds start popping, add the lentils and stir to combine.
Pour in 200ml water. Add 1/3 tsp salt and a whole green chili.
Simmer for 15 minutes, stirring occasionally.
When the lentils have softened, stir in 150ml coconut milk. Allow to simmer for 5 more minutes.
Check for consistency and add more coconut milk if you would like it creamier.
Taste for salt and adjust if necessary.