Heat oven to 180C/160C fan/gas 4. Heat a non-stick frying pan, add the black beans and fajita spice mix, stir together and cook for 1-2 mins. Add the tomatoes and simmer over a low heat for 10 mins.
Cut the tortillas in half. Lightly grease a 2-litre baking dish. Spread a third of the bean mixture over the base of the dish, followed by 3 tbsp of the cheese and a layer of tortillas. Repeat the process, finishing with a tortilla layer on top. Sprinkle over the remaining cheese. Bake for 15 mins until golden.