This Tex-Mex favorite tastes just as good wrapped in lettuce leaves as it does in flour tortillas.
In a medium bowl or resealable plastic bag, combine garlic, lime juice, cumin, 2 teaspoons salt, and 1/2 teaspoon ground pepper; add chicken, and turn to coat.
Heat 2 tablespoons oil in a large skillet over medium heat. Add chicken breasts; cook until opaque throughout, 6 to 8 minutes per side. Transfer chicken to a plate; wipe skillet with a paper towel.
Heat remaining tablespoon oil in skillet. Add peppers and onion; season with salt and pepper. Cook, tossing frequently, until vegetables are crisp-tender, 5 to 7 minutes.
Thinly slice chicken; serve with cooked vegetables and desired accompaniments, letting guests assemble their own fajitas.
The chicken can be replaced with 1 1/2 pounds flank steak, cooked in the skillet to your liking and thinly sliced.