Bring 4 quarts of water to a boil in a large pot and then add in the shells with a splash of olive oil. Boil for about 10 minutes until al dente. Drain the shells and rinse with cold water.
Mix black beans, bell pepper, white onion, mushrooms, tomatoes, jalapeno, cilantro, green onions, taco seasoning, and cheese together.
Put a layer of sauce on the bottom of a casserole dish.
Stuff the shells with about 3 tablespoons of the filling.
Spoon some sauce over the shells as well and cover the pan with foil.
Bake at 350 F for about 30 minutes.
Garnish with fresh cilantro, green onions, and cheese.
Top with salsa, guacamole, or greek yogurt (sour cream).