Heat oil in large frying pan or wok. Add onion, garlic, and peppers and cook on medium heat until they begin to soften. Add mushroom and continue cooking until veggies are tender and there is some liquid in pan. While cooking, add chili powder, salt, and pepper to taste.
While veggies are cooking, mash up black beans with back of fork or potato masher until pasty. Some whole beans can still be present. Add sour supreme by large spoonfuls and mix with beans until mixture is creamy. You may use 1/2 whole container depending how creamy you like it. Add taco seasoning to bean sour supreme mixture and stir until thoroughly combined. If you really like spice, add a little hot sauce.
Grease the bottom of a 9x13 inch casserole dish and arrange tortillas so that entire bottom of dish is covered. Tortillas can overlap, but feel free to cut them to fit. Spoon out half of cooked veggies and spread them on top of tortillas. Trying not to use liquidy part. Arrange another layer of tortillas on top of veggies and spread bean sour supreme mixture on top. Arrange another layer of tortillas and pour remaining veggies and cooking liquid on top. Using the liquidy part of the cooked veggies for the last layer allows it to seep down and flavor the whole casserole. Arrange a final layer of tortillas for top of casserole.
Place in oven and bake for about 20 minutes at 350 degrees.
Allowing casserole to cool for a few minutes before cutting will make it much easier to cut.
Enjoy!