Preheat the oven to 400 degrees.
Slice the mushrooms, onion, and bell pepper and place them on a large sheet pan.
Stir together the salt, cumin, chili powder, garlic powder, onion powder, and paprika in a small bowl to make the spice mix.
Sprinkle the fajita spice mix over the veggies and drizzle the olive oil over the top. Use your hands to toss the veggies until they are coated evenly in the oil and spices
Roast the portobello mushroom fajita veggies in the oven for 20 minutes, then stir and toss.
Roast the veggies for an additional 15-20 minutes, until caramelized.
Squeeze 1/2 of a lime over the top of the veggies before serving. Toss to combine.
Serve the fajita filling on small flour tortillas with avocado slices or toppings of choice.