Place chopped romaine lettuce into a large bowl; set aside.
DRESSING: In a small jar, combine olive oil, lime juice, brown sugar, chili flakes, and 1 teaspoon salt: Place the lid on tightly, shake until well combined; set aside.
SEASONING: In a small bowl, combine chili powder, cumin, garlic powder and 1 teaspoon salt; set aside.
CHICKEN FAJITAS: Heat the vegetable oil in a large skillet over medium-high heat. Place the chicken breast on one side of the pan, and the onions and peppers on the other side; sprinkle seasoning overall.
Sauté the onions and peppers for 4-5 minutes, stirring constantly. Flip the chicken, and cook for another 5 minutes, until chicken is cooked through and vegetables are tender. Remove pan from heat.
Thinly slice the chicken into strips. Place chicken strips, onions and peppers on top of the salad greens. Slice avocado and add to salad.
Toss everything together, serve immediately drizzled with dressing.