Set out the fish on the kitchen counter to defrost and proceed with the steps listed below. By the time you gather all the ingredients and get to to the fish, it should be somewhat defrosted and ready for you.
Blend together the onions, tomatoes ginger, garlic, and 1/2 cup of coconut milk. Pour this sauce into the inner liner of the Instant Pot.
Stir in the salt, cayenne, turmeric and garam masala. Use the 1/4 cup of water to wash out the blender jar and pour that last bit of goodness into the pot.
Add in the frozen spinach and stir. The spinach or greens are what makes this a saag.
Place a steamer rack on top of this sauce.
For the Fish: Rub the fish cubes with oil, salt, and turmeric. Place the fish on a sheet of aluminum foil and fold the sheet over on all sides to make a securely-wrapped package. Place this package on top of the steamer rack.
Cook for 5 Minutes high pressure.
Allow it to release pressure naturally for 5 mins and then release all remaining pressure.
Very carefully lift up the package of fish, open one side of it, and tilt it so that all the cooking liquids pour into the sauce below. Set the package of fish aside. (You are doing this so that you don't break up the fish as you proceed to the next step which asks you to mix the sauce well.)
Pour in the reserved canned coconut milk and stir well until it is well incorporated. Only once this is done will you carefully open the package of fish and tilt the pieces of fish into the spinach saag. Serve with rice, or naan, cauliflower rice, or zoodles.