Set foodi to saute and add butter and onion; cook 5 minutes stirring frequently, then remove from pot and allow to cool.
Add the ground beef, eggs, breadcrumbs, milk, parsley, ketchup, garlic, italian seasoning, pepper and cooled onions to a large bowl and mix until fully combined.
Transfer meatloaf mixture to a double layer of aluminum foil, then shape into a loaf and fold the edges of the foil up to make a foil loaf pan around the meatloaf.
Place the water and the trivet into the bottom of the foodi pot, then carefully transfer the meatloaf in the foil on top.
Place lid on, set the valve to sealed, then pressure cook high for 35 minutes; and allow a 10 minute pressure release before venting.
Meanwhile, prepare the glaze by stirring together the ketchup, brown sugar, and apple cider vinegar.
Check that the internal temperature is at least 165 F,* then brush the topping onto the cooked meatloaf.
Use the broiler function to broil for 5 minutes, or until topping is bubbly.
Sprinkle with chopped parsley and serve warm.
Notes
* If your meatloaf has not reached 165 degrees F, place the lid back on the Foodi and bring it to high pressure for another 5 to 10 minutes depending on doneness. You can also remove the meatloaf from your Ninja Foodi and finish it in the oven, cooking it at 375 degrees until done.