Pop your onions, mixed peppers and chicken breasts down on a large baking tray. Toss in the oil, salt and fajita seasoning. Shove in the oven for 30 minutes at 375ºF. Bring out and leave to cool slightly.
Slice your chicken breasts thinly and set aside.
Line a 24cm springform cake tin with baking parchment and warm your tortillas briefly. Pop one in the base then add some peppers/onions, then chicken, then black beans, salsa and cheese. Pop another tortilla on top and repeat the process until everything is used up.
Fold the edges of the final tortilla in on itself to create a neat lid.
Cover with foil and bake for 15 minutes at 400ºF.
Uncover, bring out and leave to cool slightly.
'Ice' the top with sour cream and decorate with chilli and coriander (if using).