Prepare all ingredients as listed.

Heat the olive oil in a large skillet over medium heat, then add the onion, red bell pepper, and green bell pepper, season with salt and black pepper, and sauté for 5–7 minutes until the vegetables are tender and the onion is translucent.

Add the sliced chicken to the skillet, sprinkle with garlic powder, cumin, paprika, and additional salt and pepper if desired, and cook while stirring occasionally for 6–8 minutes, until the chicken is fully cooked and reaches an internal temperature of 165°F (74°C).

Mash the avocado in a bowl with a fork, then add cilantro, lime juice, garlic powder, salt, and black pepper and mix until smooth or slightly chunky; cover and refrigerate until ready to serve.

Warm the soft taco shells in a dry non-stick pan over medium heat for 20–30 seconds per side until soft and pliable.

Add the lemon slices to the sparkling water, add ice if desired, and serve chilled.

Serve the chicken fajitas with warm tortillas, sour cream if using, tortilla chips with guacamole, and lemon sparkling water.