Clean the peppers and cut them in half. Cut each half into 4 long slices. Then cut each one of those slices diagonally.
Slice the onion in a half moon and the chicken into strips for fajitas.
Add the soy sauce, salt, pepper, paprika, cumin, and garlic salt onto the fajitas and mix until evenly combined. Set aside for 10 to 15 minutes so the chicken is marinated.
Heat a saucepan or large skillet and add the oil. Add the onion and cook for 3-4 minutes before adding the chicken. Cook for about 5 minutes or until it's no longer pink.
Add in the bell peppers and mix until combined. Let them cook for another 20 - 25 minutes until the peppers are softened.
Serve immediately over corn or flour tortillas.
Garnish with avocado, guacamole, and cilantro.