Measure the steel cut and oats and add about 2 cups of water to it and cook until the oats are soft. I used Instant pot to cook my steel cut oats. I set it on manual for 6 minutes and it cooked perfectly. You could cook it on stove top as well. Make sure you stir it often to avoid sticking to the bottom of the pan. It takes a good 20 minutes to cook the steel cut oats. Add more water if needed.
In a heavy bottom pan, measure the jaggery and add about a cup of water to it. Let it come to a boil and then strain the mixture to remove any impurities. Return the jaggery to the pan and bring it to a boil again.
Mash the cooked oats and add it to the jaggery and mix well. The mixture will liquefy a bit. SImmer the flame and let it cook in the jaggery syrup for 5 to 7 minutes.
Meanwhile, if using canned coconut milk, measure half a can of coconut milk and pour it in a separate bowl. Add about 1 cup of water to it. This becomes the thin coconut milk. The other half in the can will remain the thick milk. If using fresh coconut to remove milk, measure about 1/2 cup of thick milk and then use about 2 cups of thin milk.
Add the thin coconut milk to the pan and let it simmer and come to a gentle boil. Let it cook for 5 to 7 minutes.
Now add the thick coconut milk and bring it to gentle simmer. Do not boil too long after you add the thick milk as the mixture might curdle and separate.
Heat the coconut oil in a small pan and roast the cashew nuts and the grated coconut. Add this along with cardamom powder to the payasam and mix well. Serve warm or at room temperature.