Cut the stem off of the mandarins and slice the mandarins in half. Slice each half into slices about ¼" thick. Throw them in the inner pot of the Ninja Foodi or your pressure cooker
Add 2 cups of water and 2½ cups of white sugar. Stir.
Put the pressure lid on the Ninja Foodi and turn the valve to seal. Select high pressure for 10 minutes. When the time is up, allow the pot to naturally release its pressure. This takes about 25 minutes.
Open the lid and turn on the sear/sauté on high. Bring the mixture to a boil and add in remaining ½ cup of sugar. Bring back up to a boil and boil until the mixture reaches about 221°F-223℉ (105-106°C). This is important in order for your marmalade to set up. I usually stir occasionally during this process to avoid scorching. The best way to determine if your marmalade is done is by using a thermometer. However, there is a change in the appearance of the bubbling if you don't have a thermometer. See post for details and short video.
Once the orange marmalade has reached at least 221°F, turn off the heat and stir. Allow to cool and then ladle into jars and let them sit at room temp for 1-2 hours, then refrigerate at least 4 hours to set up.