Combine cumin, chili powder, garlic powder, dried oregano, salt, and cayenne pepper (if using) in a small bowl to create your fajita seasoning.
Pat chicken breasts dry and rub with 1 tablespoon olive oil, then coat with two-thirds of the fajita seasoning mixture. Let marinate for at least 15 minutes.
Heat a grill pan or skillet over medium-high heat. Cook the chicken for 5-7 minutes per side until internal temperature reaches 165°F (74°C). Remove and let rest for 5 minutes before slicing.
In the same pan, add the remaining tablespoon of olive oil. Add sliced bell peppers and red onion, sprinkle with remaining fajita seasoning. Cook for 4-5 minutes until slightly charred but still crisp.
Wash and chop romaine lettuce into bite-sized pieces and place in a large serving bowl.
Slice the rested chicken across the grain into thin strips.
Arrange the grilled chicken strips and pepper-onion mixture over the bed of romaine.
Sprinkle freshly chopped cilantro over the entire salad.
In a small bowl, whisk together fresh lime juice, a tablespoon of olive oil, and a pinch of salt. Drizzle over the salad.
Add any optional toppings as desired—sliced avocado, cherry tomatoes, black beans, corn, or cheese.
Gently toss everything together just before serving and serve with additional lime wedges.
