Prepare ingredients as instructed.
Cook vegetables: Heat olive oil in a large skillet over medium heat. Add onion, red bell pepper, and green bell pepper. Season with black pepper. Sauté for 5–7 minutes, until onions are translucent and peppers are tender.
Cook chicken: Add sliced chicken to the skillet. Season with garlic powder, ground cumin, paprika, and black pepper if desired. Cook, stirring occasionally, for 6–8 minutes or until fully cooked and the internal temperature reaches 165°F (74°C).
Make the guacamole: Scoop the avocado into a bowl and mash with a fork. Add cilantro, lime juice, garlic powder, and black pepper. Mix until smooth or slightly chunky. Set aside.
Heat olive oil in a saucepan over medium heat. Saute the garlic and onion until the onion begins to soften. Stir in the ground cumin, then add the black beans and water. Season with salt and pepper and low simmer for about 15–20 minutes to develop flavor. Remove from heat and stir in chopped cilantro.
Warm taco shells: Heat a dry non-stick skillet over medium heat. Warm soft taco shells for 20–30 seconds per side until soft and pliable.
Prepare lemon sparkling water: Fill glasses with ice cubes. Add sparkling water and lemon slices.
Serve chicken fajitas with cheese, taco shells, and greek yogurt, if using. Serve with tortilla chips, guacamole, and lemon sparkling water.