Preheat the oven to 425°F (220°C). Line two rimmed baking sheets with parchment paper.
In a small bowl, combine the avocado oil, lime juice, chili powder, garlic powder, cumin, oregano, salt, and black pepper. Add the cayenne pepper, if desired, for spicier fajitas and stir to combine.
Scatter the bell peppers and onion on one of the prepared baking sheets. Drizzle with 2/3 of the oil mixture, and using your hands or tongs, toss to coat. Spread the vegetables in a single layer.
On the other baking sheet, toss the steak with the remaining dressing and arrange in a single layer.
Transfer the sheet pan with the vegetables to the oven and roast for 5 minutes. Place the sheet pan with the steak in the oven and continue to cook until the steak is done to your liking and the vegetables are just tender, 9 to 10 minutes more.
To warm the tortillas, place a large skillet over medium-high heat. When hot, add one to two tortillas and cook 30 seconds per side. Transfer the tortillas to a plate and cover with a clean kitchen towel to keep them warm. Repeat with the remaining tortillas.