Cook up the mushrooms by heating a medium-size skillet over medium/high heat and add 2 tablespoons of olive oil and 2 tablespoons of butter.
Add in the mushrooms with a sprinkle of salt.
Cook the mushrooms until they become slightly golden. At some point during the cooking, add in the other 2 tablespoons of olive oil.
Stir in the taco seasoning and let the mushrooms cook for another minute with the seasoning.
Stir in the lime juice and cilantro, then remove the pan from the heat.
Make the veggies by heating a large skillet over medium/high heat.
Add in about 2 tablespoons of olive oil and add in the onion and bell pepper with a sprinkle of salt.
Cook until softened, adding the other couple tablespoons of olive oil as needed.
Stir in the taco seasoning and let the veggies cook for another minute or so with the spices, then remove from heat.
Assemble the tacos. For a single taco, add 1 teaspoon of butter to a small or medium skillet, and place the tortilla on top of the melted butter. Top the tortilla with the shredded cheese and allow the cheese to melt. Once melted, add in the mushrooms and veggies.
Once the tortilla has started to brown on the bottom, fold in half to create a taco. Add toppings and serve.
I like to use these taco holders to make the whole process easier.