Heat a dash of oil in a large frying pan, and add the onion, garlic, pepper and mushrooms. Cook over a medium heat for 5 minutes, stirring regularly, until fairly soft. Add the kidney beans, and cook for a couple more minutes.
To make the homemade enchilada sauce, heat a couple of tablespoons of oil in a small saucepan, and add the flour and spices. Cook over a low heat for a couple of minutes, stirring constantly, until fragrant. Add the tomato puree and vegetable stock, and whisk constantly until the mixture comes together into a smooth sauce. Simmer for a few minutes until the sauce reaches your desired consistency (add a dash more water if you'd like it to be thinner).
To assemble the dish, add one small spoonful of the sauce to the bottom of a baking dish, and spread it around with the back of a spoon (this is just to stop the tortilla from sticking). Add a small flour tortilla, and top with some of the vegetables, cheese, and one spoonful of the enchilada sauce. Repeat with the remaining ingredients, layering upwards until you've used all of the veggies (the exact order and quantities doesn't matter - I ended up using 5 tortillas in total, so 4 layers of veggies and cheese in between). Finish the dish by pouring over any remaining sauce, and topping with any remaining cheese. It doesn't matter if you don't entirely cover the tortillas around the edges.
Bake at 190°C (Gas Mark 5 / 375°F) for around 25 minutes, or until the cheese is crispy and the sauce is bubbling.
Serve with plenty of fresh toppings - I added fresh coriander (cilantro), tomatoes and avocado on top, and served a dollop of sour cream on the side.