Slice chicken and place in a large bowl.
Add olive oil, lime juice, vinegar, and fajita seasoning. Toss well to coat.
Cover and refrigerate for at least 30 minutes or up to all day.
Heat cast iron pan on high heat. Cook chicken until browned, about 4–5 minutes per side.
Add onions and peppers; sauté until softened and slightly charred.
Serve over rice with guacamole, pickled red onions, and a drizzle of hot sauce.