Heat a medium/large saute pan over medium-high heat and add the oil.
Once the oil is hot, add the onion and saute until translucent and beginning to brown, about 3 to 4 minutes.
Once the onion is cooked, add the red bell pepper and mushrooms.
These vegetables will release a lot of water as they cook.
Cook until most of the moisture has evaporated, about 5 to 7 minutes.
Add the black beans, taco seasoning, worcestershire sauce, water and salt.
Simmer for 5 to 7 minutes, stirring occasionally.
Prepare the ingredients for the toppings (cheese, lettuce, tomato, and cilantro) while the filling is simmering.
Lightly mash some of the black beans while simmering. Add more water if needed to keep the taco filling from drying out.
When the taco flling is done, turn off the heat and set aside.
Toast the tortillas (see Recipe Notes).
Assemble the tacos by dividing the filling evenly between the tortillas, and top with lettuce, tomato, and cilantro.
One taco should have about 1/4 cup of filling and 1 Tablespoon of cheese.
