Prepare the mushrooms by removing stems and, if desired, spooning out the gills; wipe caps clean and slice into ~1/3-inch thick pieces.
Thinly slice the bell peppers and onion and set aside.
Heat a large skillet over medium heat and add 1 Tbsp olive oil (or 1–2 Tbsp water or broth for oil-free cooking).
Add the sliced peppers and onion to the skillet and cook, stirring, for about 2 minutes until they begin to soften.
Add the sliced mushrooms and 1 Tbsp taco seasoning to the skillet, stir to combine, and cook for another 7–8 minutes until the mushrooms and peppers are softened, stirring occasionally.
Stir in 1 Tbsp fresh lime juice, taste and add salt if desired, then remove from heat.
Warm the corn tortillas if desired, then spoon the fajita mixture into the center of each tortilla and top with your choice of avocado, guacamole, salsa, vegan cheese, vegan sour cream, or nutritional yeast.