Combine flour, sugar, baking powder and salt in a large bowl. Make a well in the center and pour in milk, oil and egg. Mix until smooth.
Take out the airfryer-pan and pour the mix in circles onto the pan. Cook until bubbles form and the edges dry(1-2 minutes) Flip over and cook until browned both sides. Repeat with the rest of the batter.
Peel and slice a red onion 6 mm slices. Clean button mushrooms; then cut into 6 mm slices. In a sauté pan with olive oil, sweat onions and mushrooms under a medium flame until tender. Remove from heat and place on a dry kitchen towel.
Preheat the airfryer to 200 degrees. In a mixing bowl, crack 6 eggs and whisk thoroughly and vigorously, adding a pinch of salt. In a 20 cm heat resistant baking dish, coat the inside and bottom with a light coating of pan spray. Pour eggs into the baking dish, then the onion and mushroom mixture and then the cheese.
Place the baking dish in the cooking basket and cook in the airfryer for 27-30 minutes. The frittata is done when you can stick a knife into the middle, and the knife comes out clean.