Place aluminium foil underneath a 7.5in x 3in inch springform cake tin and wrap around. This will prevent leakage. Cut a round sheet of baking paper and fit it onto the inside base of the cake tin. Then place it into your air fryer.
Lightly spray the sides and base of the cake tin with cooking spray and evenly add your onion, bacon and salmon to the cake tin. Cook for 6 minutes at 180°C (350°F).
While onion, bacon, and salmon are precooking, crack six large eggs into a bowl. Then add 3 tbsp of milk together with a pinch of salt and black pepper. Whisk until well combined.
When the air fryer has finished precooking, pour in the beaten eggs, and add the grated cheddar and mozzarella cheese. Then add chopped tomato and spinach leaves, ensuring all ingredients are evenly spread in the egg.
Set the temperature of your air fryer to 150°C (300°F) and cook for 24-26 minutes or until golden brown. (See note 1)Use a wooden skewer to check if the egg has set in the centre if you're unsure.
Remove the frittata from the cake tin and rest for 5 minutes, slice and serve immediately. Optionally garnish with parsley or freshly chopped spring onion.
Note 1: If you find that the top of the frittata is overly brown before the egg has set, place a sheet of aluminium foil over the top and cook for a further 5 minutes.