In a large bowl toss together onions, peppers, and mushrooms with oil and spices. Distribute on the large baking sheet. Roast for 25 minutes, tossing halfway through, until veggies are soft to fork and slightly browned at the edges.
Heat tortillas in a hot pan over medium heat, if desired. Divide the portobello mushroom fajita mixture into 6 tortillas. Garnish with cilantro. Serve with salsa, avocado, shredded cheese, shredded cabbage, and a squeeze of lime. Enjoy!