Make the tacos: Slice the steak into ¼-inch (6-millimeter) thick strips.
Add the steak to a medium bowl with the taco seasoning and toss to coat. Let marinate in the refrigerator for 1 hour.
Preheat a large 12-inch (30-centimeter) cast iron skillet over high heat. When the pan is hot, add the oil. Working in batches, cook the steak for about 2 minutes per side, or until the meat is browned and just cooked through. Remove from the pan and repeat with rest of meat.
To the same pan, add the onions and bell peppers, then reduce the heat to medium. Cook the vegetables until tender. Season with salt, then add the meat back to the pan and toss with the vegetables.
Serve the fajitas in the pan with tortillas, shredded cheese, salsa, and avocado alongside.
Make the next-day grain bowl: Reheat the leftover fajitas in a medium pan over medium-high heat until warmed through.
Divide the cooked quinoa between 2 bowls. Top with the fajitas, black beans, shredded cheese, avocado, and cilantro.