In a bowl, whisk all the marinade together; about 3 tablespoons of olive oil, cilantro, lime juice, garlic, red pepper flakes, chili pepper, brown sugar (if you’re using), cumin, salt, and pepper. Set aside a small quantity of the marinade and if you would like to drizzle over the chicken when you're done.
Heat the remaining oil in a skillet or a pan over medium-high heat and cook the chicken, you might need to cook in batches. Cook on both sides for about 3-5 minutes or until chickens are cooked. For this step, you can marinate and cook the chicken breast without cutting then cut afterward. Transfer into a plate.
In the same pan, add the bell peppers and onion, stir season with salt and pepper and cook for about 2-3 minutes or until cooked. You can also add some of the marinade you reserve on the vegetables while cooking if you desire.
Serve chicken with the pepper mixture, avocado, lettuce, corn and enjoy. For some acidity add some lime.
Enjoy!