Preheat a 12 inch cast iron skillet over medium-high heat. Add in oil, onions, and peppers. Saute for 4-5 minutes or until onions begin to soften and caramelize.
Add in garlic, portobello mushrooms, and spices. Saute for 4-5 minutes longer until mushrooms soften and the liquid that was released into the pan mostly renders off (it will not all render off and that is okay!)
Turn off the heat and stir in the zest and juice of one lime.
Use a slotted spoon and serve on top of your favorite tortillas and top as desired.
Cast Iron skillets are best for authentic fajitas. The char that is achieved on the peppers and onions in a cast iron skillet is hard to beat. However, you can use a nonstick skillet if that is all you have. Portobello Mushrooms: Simply wipe the mushrooms down with a damp towel or paper towel to remove the dirt and then slice. I don't remove the gills from the mushrooms, as I like the meaty taste it gives these fajitas. But if the texture bothers you, by all means, scrape the gills from the mushrooms before slicing them. Oil-Free Option: To keep these plant-based fajitas oil-free, saute the vegetables using vegetable broth instead of oil. Start by placing 2 tablespoons of broth in the skillet and heating the broth until sizzling. Add in the onions and peppers and proceed with the recipe, adding more stock as needed. Watch closely to not burn the vegetables. Leftovers: Store leftover fajita mixture in an airtight container for up to 4 days in the refrigerator. It is best to reheat the mixture in a dry, hot skillet so the vegetables do not become soggy, as they would if heated in the microwave.