In a large bowl, beat together the eggs, salt, and several grinds of black pepper. Fold in the chopped broccoli, green onions, and cheddar.
Generously grease a 7-inch pan, then pour the egg mixture into it. Pour 1 cup of water into the bottom of your Instant Pot and place a trivet over that to keep the pan above the water.
Place the pan with the frittata mixture on top of the trivet and secure the lid. Move the steam release valve to Sealing. Use the Pressure Cook or Manual button on your machine to cook at high pressure for 10 minutes.
When the cooking cycle is complete, let the pressure naturally release for 10 minutes. (In other words, don't do anything for 10 more minutes.) When the screen reads LO:10 you can move the steam release valve to Venting to release any remaining pressure.
When the floating valve in the lid drops, it's safe to remove the lid. Use oven mitts to remove the frittata pan and don't be alarmed if you see liquid on top of the baked eggs. It will evaporate quickly as it cools.
Slice the frittata into 4 slices and serve warm. Leftovers can be stored in an airtight container for up to 5 days in the fridge.