Parcook the beets until they are soft enough to slice, but still firm to bite. My favorite way to cook them is with the peels on in my pressure cooker. I pressure cook them in the basket or on the rack with 1 cup water on high for 5 minutes with an immediate release. When done, put them in a bowl of cold water and rub the outside with your hands and the skins will slip right off.
Preheat the air fryer on its hottest setting. For the Ninja Foodi Pressure Cooker & Air Crisper this is the Broil (Grill in the UK) setting for a full 10 minutes. Make sure the basket or cooking surface is in the appliance so it preheats as well. This will prevent sticking.
If you refrigerated the beets, let them come to room temperature before or after slicing for the best results. Slice the beets, using a sharp knife or mandoline slicer, into ⅕-¼" slices. ⅕" is what I aim for. Any thinner and they tend to stick together, any thicker and they brown before crisping.
Place the beets into a mixing bowl and drizzle with oil. Season with salt.
Place the beets into the air fryer basket and set the air fryer to 300°F for 20 minutes. After 3-5 minutes, use tongs to toss around the beet chips and gently separate any that are stuck together. Repeat this process every 5 minutes until the beets become a bright red and lose most of the shiny dark red areas. You may see some browning, but don't let them get too brown or they will taste burnt.
Remove the beet chips from the basket and allow to cool on a cooling rack. Enjoy!
The beet chips can be stored in an airtight container once they are completely cooled. If left exposed to humidity for any period of time, they will soften. You can pop them back into the air fryer for a few minutes to crisp back up, but be careful not to burn them.