Pour 1 1/2 cups water into the Instant Pot. Coat a round 6- or 7-inch cake pan with cooking spray.
In a medium bowl, combine berries and sugar. Let sit for 5 minutes. Pour into the prepared pan.
In a medium bowl, combine flour, oats, brown sugar, cinnamon, nutmeg and salt. Mix well. Add the melted butter and stir until crumbly. Pour evenly over the fruit and cover the pan with foil. Place on trivet and lower into the pot.
Place the cover on the pot and set timer to 10 minutes on high pressure (pressure cook setting). It will take the IP around 8 minutes to build up pressure before it begins cooking.
When cooking is done, let the pressure release naturally for 5 minutes before doing a quick release. Allow the float valve to drop.
Remove cover and pull the trivet/pan out. Remove foil and serve with ice cream and whipped cream, if desired.