Preheat your oven to 425°F (220°C).
Slice the red onion into thin wedges and the bell peppers into strips. Remove the stems from the portobello mushrooms and cut them into thick slices.
In a small bowl, combine the fajitas spice mix ingredients: sweet smoked paprika, ground cumin, garlic powder, oregano, salt, pepper, and hot chili powder or flakes. Mix well and set aside.
On a large baking sheet, spread out the sliced onion, bell peppers, and portobello mushrooms. Drizzle with olive oil and sprinkle the spice mix over the top. Use a spatula to toss everything together.
Place the baking sheet in the preheated oven. Roast for 20-25 minutes, stirring halfway through.
Warm your tortillas in a dry skillet over low heat for about 30 seconds on each side or wrap them in foil and place them in the oven for the last 5-10 minutes.
To assemble, place a generous amount of the roasted mixture onto each tortilla. Top with guacamole, salsa, sour cream, and black beans.
Serve your Sheet Pan Portobello Mushroom Fajitas hot, garnished with extra toppings of your choice.