Dice the onion into ½" dice. Turn the Ninja Foodi on High Sear/Saute. Add butter and onions, saute for about 5 minutes. While the onions are sauteing, cut up your potatoes into ½" dice and add to the pot.
Crack 12 eggs into a large mixing bowl. Using an Immersion blender or whisk, blend eggs. Cube the ham into bite size pieces, about 1" in size.
Turn off the Ninja Foodi and dump in the eggs and the heavy cream. Add the ham and the cheese. Stir to combine. Put the pressure lid on and turn the seal to vent.
Set the Slow Cooker function to low for 8 hours.
You can serve the breakfast casserole just as it is or top it with the remaining cup of cheddar cheese and broil for about 5 minutes or until the cheese is browned like you like it. Serve and Enjoy!
Don't worry about going over the 8 hours slow cook time. The Ninja Foodi will automatically switch to keep warm and even after 12 hours, the breakfast casserole was perfect!