Whisk the eggs, mustard powder and salt in a bowl or large mixing jug [Note 2]. Add the shredded cheese and the other filling ingredients and mix until combined.
Preheat air fryer to 320°F / 160°C.
Carefully pour the frittata mixture into the pie pan. Do not over-fill it, be sure to leave at least ¼ inch of space at the top as the frittata will expand as it cooks.
Place the frittata in the air fryer basket (take care as air fryer basket will be hot). Air fry frittata for: - 20 minutes for individual frittatas - 25 minutes for a single 8inch/20cm frittata
Frittata is done when the top is golden brown, and it has reached 160°F / 72°C in the center [Note 3].
Leave the frittata to set for at least 10 minutes [Note 4].
Notes
You will need either 4x 4½ inch/ 10-12cm pie pans, or a single 8 inch/ 20cm pie pan.
Use a mixing jug if you have one large enough as it makes it much easier to pour the filling into the pan when it is ready.
Check with an instant-read thermometer.
Leave it to set for longer if you want to cut nice clean slices.