First, heat oil in a large pan and add the chopped onion, garlic and bell peppers. Saute until the onion turns translucent.
Now,add the sliced mushroom to the pan. Add the salt, red chilli flakes and mix well. Cook in medium flame. The mushroom will release water initially. Continue to cook until all the water evaporates and the mushrooms are dry and 80% cooked.
Now add the salsa and corn to the mushroom. Mix well and continue to cook for a couple of mins until the salsa warms up and thickens a bit. Turn of the stove.
Preheat the oven to 400 F. Take a 8X8 baking pan. Cut the tortillas as per the size of the pan. Place 2 tortillas in the bottom of the baking pan. This would form the base of the casserole. Add half of the mushroom mixture on top of the tortilla and spread evenly.
Now, add half of the sour cream on top of the mushroom mixture and spread it out gently. On top of the sour cream, sprinkle half of the shredded cheese. Repeat the same process for a 2nd layer of tortilla, mushroom mix, sour cream and cheese.
Bake in the 400 F for about 20 mins until the top is nicely melted and golden. Pull it out of the oven and let it sit for 10 mins.
Loosen the edges with a sharp knife and cut into quarters like a lasagna. Serve hot.