Olan is a very modest curry from Kerala made with Ash Gourd (White Pumpkin), Yellow Pumpkin, Black-Eyed Peas/Vanpayar (Chowli) OR Red Cow Peas cooked in Coconut Milk.
My usual combo for Olan is White Pumpkin and Black Eyed Peas/Chowli. The flavour of Olan depends of the thickness of coconut milk. 
Olan - Onam recipe

Olan is one of the essential items for Onam/Vishu, yet it’s so humble that you may not even notice Olan in the corner of the banana leaf (in a sadya/feast). I make Olan on those days when we had a heavy lunch and craving for a light dinner, or been on vacation – outside food overdose and craving for light home food, easy on the stomach and easy on efforts. Olan can be done real fast.. no grinding, no elaborate cooking; just nothing.
It’s a simple curry with a beautiful flavour of just one thing – my favourite – coconut milk. 🙂  Authentically, Turmeric Powder is not added to this recipe; and seasoning (tadka) is also not given to Olan; just coconut oil (it’s a Kerala dish guys!) drizzled on top.
The thing with Black Eyed Peas/Chowli is that it’s ‘ok’ even if you forget to soak it beforehand. Just soak for 15-30 minutes in hot water and pressure cook upto 3-4 whistles. It will cook to perfection. Although I never forget, nor make this one unplanned 😛
How to make Coconut milk: 
Grind grated coconut with sufficient quantity of water and squeeze out the milk to a bowl. Repeat the process again until the milk starts thinning. Fresh coconut milk is ready!
I take all this efforts only on special occasions due to –  one, because I am lazy; two, because I hardly find the time and patience!  So mostly I have a stock of coconut milk  powder.  Heat a cup of water and add in 2 tbsps of coconut milk powder. Mix well and your coconut milk is ready. (use more of the powder for thick milk and less for thin diluted milk).
Ingredients:
Ash Gourd/Pumpkin – ¼ Kg – cut into thin square slices or small cubes
Black Eyed Peas/Chowli – 3/4th cup (before soaking)
Coconut Milk powder – 2 tbsp mixed in 1 cup hot water
Curry Leaves – 8
Green Chillies – 3 slit
Coconut oil – 1 tbsp
Salt
Before you begin:
Soak Black Eyed peas overnight.
How I prepare:
  1. Boil Ash Gourd in water till tender.
  2. Pressure cook soaked Black Eyed peas with water just to immerse it. Allow the pressure to subside on its own. Then add the cooked Black Eyed Peas/Chowli the cooked Ash gourd.
  3. Now, there may be excess water when you cooked the peas. If this water is added to the ash gourd, the curry will become very thin. So, what can be done is, either throw the water (heart-breaking) or dissolve coconut milk powder in this water(when hot) instead of fresh hot water.
  4. Add salt, curry leaves and slit green chillies.
  5. Boil for a few minutes and add coconut milk. After adding coconut milk, do not boil for a long time, just bring it to a boil on simmer and turn off the stove.
  6. Pour 1 tbsp coconut oil on top and it’s ready to serve with rice.

Olan is also an option to serve during sickness since it has minimum masala/spice.
Another one of my coconut milk recipe: Potato Stew
    Olan - Olan - Kerala recipe with ash gaurd