In a small bowl, mix together the smoked paprika, onion powder, black pepper, cumin and 1/2 teaspoon salt. Set aside.
Remove the stems from the mushrooms. You can remove the gills if you want to but it's not needed (I don't).
Cut the caps into 1/2" slices. You can also cut up the stems and use those.
Once all the vegetables are prepared, preheat a 12" cast-iron skillet over medium-high heat.
Once it's very hot, add 1.5 tablespoons of olive to the pan.
Add a mushroom slice. It should immediately start to sizzle. If it doesn't, wait a bit and try again. Once you know the pan is really hot enough, add the mushroom slices. They should be in 1 layer - it's okay if a few are stacked on top of the others. You might need to do this in 2 batches, depending on how big your mushrooms are. If doing them in 2 batches, add another 1.5 tablespoons of oil to the pan.
Cook the mushrooms for 2-4 minutes, without stirring, until browned on the bottom. If you think they're starting to burn, you can flip them and turn down the heat.
Flip over each mushroom slice and let them cook for another 2-4 minutes. They should have shrunk and be nicely browned. Sprinkle 1/4 teaspoon (if doing 2 batches, then use 1/8 teaspoon over each batch) salt on top, stir to coat the mushrooms, and place the mushrooms in a bowl.
Add another 1.5 tablespoons of olive oil to the hot pan.
Once it's very hot, add the bell pepper and onion. Sprinkle the spice mix over the top and stir well. Let sit about 2 minutes or until browned then stir again. Continue cooking, stirring occasionally, for another 4-6 minutes or until the vegetables are browned and tender to your liking. When it's almost done cooking, add the garlic. You don't want to add it too soon or it might burn. Turn the heat up or down as needed. There will be some bits stuck to the pan but you'll be able to deglaze that when you add the lime juice.
The bowl of mushrooms might have a bit of water in it. Drain off the water and add the mushrooms to the pan. Stir to combine them with the other vegetables.
Sprinkle the lime juice over the vegetables, stir to combine, and saute for another minute. Taste and add more salt, if needed. I used a wooden spatula to scrape up the browned bits that are stuck to the pan. They have a lot of flavor!
Place the fajitas in the tortilla (or lettuce) and add your toppings.
Refrigerate any cooled leftovers in an airtight container for up to 4 days.