Place a metal trivet or round cake cooling rack in the bottom of the pressure cooker pot. Pour in water.
Tightly wrap a 6- or 7-inch cake pan with removable bottom with aluminum foil. Let excess hang over the top, or create a sling from more foil, to remove pan later. Coat inside of pan with nonstick spray.
Whisk together eggs, milk, chives, salt, and pepper.
Put the cheese into cake pan, then pour the egg mixture over the top. If needed, stir a little to make sure the ingredients are evenly dispersed.
Place the pan on the trivet inside the pressure cooker. Lock the lid in place. Select High Pressure, then 30 minutes, making sure the valve in the lid is closed.
When the pressure cooker beeps, wait 10 minutes, then turn the lid valve for a quick pressure release. Carefully open the lid, remove the pan, and let cool slightly before setting on a can or jar to lift the bottom up out of the pan.
Slice and serve, or cool completely and reheat individual slices in the microwave.