Once the oil is hot, drop in the curry leaves and fry for 30 seconds.
Add in cumin, fennel and mustard seeds in oil another 30 seconds.
Add in sliced onions, green chilies and saute for about 90 seconds
Transfer half of this mixture to a bowl and set aside.
In the remaining mixture, add lentils, turmeric, water and salt to the pot/wok.
Bring to a boil and keep the fire on medium heat, cover with lid, and cook until lentils are very soft and fall completely apart. Stir occasionally and If the dal is too thick, feel free to add more water to your desired consistency.
Add in the remaining half of the saute ingredients.
Notes
Traditionally dal has a thicker stew-like consistency but if you prefer it thinner, you just need to add more water. Make sure to stir the dal once in a while when cooking, so it does not burn at the bottom.