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 5-7 minutes.

Add the beef and cook until pink is no longer visible, then add the black beans, water, salt, taco seasoning, and Worcestershire sauce.

Simmer for 5-7 minutes, stirring occasionally. Add more water if needed to keep the filling from drying out. Prepare the ingredients for the toppings (cheese, lettuce, tomato, and cilantro) while the filling is simmering.

When the taco filling is done, turn off the heat and set aside.

Toast the tortillas. This can be done in multiple different ways. One way is to heat small stainless steel pan over medium-high heat and toast the tortillas one at a time until slightly charred around the edges, about 30 seconds - 1 minute per side. If you have a gas stove, you can use tongs to place tortillas directly over a low flame, turning when brown spots begin to form on the tortillas.

When finished toasting, wrap the tortillas in a cloth napkin or tin foil to keep warm.

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 Tbsp of cheese.