Heat a large sauté pan over medium heat and add the olive oil.
Season the chicken strips with salt and pepper, then add them to the hot pan and cook undisturbed for about 2 minutes to brown.
Add the red bell pepper strips and sliced white onion to the pan with the chicken.
Continue to cook, stirring occasionally, for 6 to 8 minutes more until the chicken is cooked through and golden and the vegetables are tender-crisp.
Remove the pan from heat and stir in the chopped cilantro.
Spoon portions of the chicken, peppers, and onions into each iceberg lettuce leaf.
Top each lettuce fajita with salsa and a dollop of non-fat Greek yogurt if using, then serve immediately.