Clean the peppers and cut into thin strips, then halve the strips again. Clean the spring onions and cut into thin rings.
Heat oil in a deep pan or wok over medium-high heat. Fry the chicken strips for 5 minutes, stirring occasionally, until the internal temperature reaches 74°C (165°F). Season with salt, pepper, and chili powder to taste.
Add the peppers and spring onions to the pan and fry for 5 minutes. Meanwhile, wash and dice the tomatoes. Add them to the pan and fry for 2 minutes more.
Clean the iceberg lettuce, cut into strips, and place in a bowl. Put the salsa, sour cream, and grated Gouda cheese into separate bowls. Arrange all bowls on the table along with the pan of chicken and vegetables.
Warm the wheat tortillas according to package instructions and place on the table. Assemble fajitas by filling each tortilla with the chicken mixture, lettuce, and toppings from the bowls as desired.