Preheat the oven to 375 degrees F (190 degrees C).

Heat olive oil in a large skillet over medium heat. Add onion, green bell pepper, and red bell pepper; cook, stirring occasionally, until just softened, about 2 minutes. Remove with a slotted spoon and set aside.

Add chicken to the same skillet and cook until no longer pink on the outside, about 3 minutes. Return onion and bell peppers to the skillet. Stir in water and fajita seasoning. Cook and stir until the chicken has completely cooked through.

Arrange 6 tortillas around edges of a half sheet pan so half of each hangs over the sides; place 1 tortilla in the center to cover the bottom.

Spoon chicken mixture evenly over tortillas, avoiding excess liquid. Sprinkle evenly with cheese.

Place 1 tortilla in the center on top; fold overhanging tortillas toward the center to enclose filling. Top with a second baking sheet to press.

Bake in the preheated oven for 10 minutes. Remove the top baking sheet and continue baking until the tortillas are golden and crispy, 8 to 12 more minutes.

Slice into rectangles and serve topped with sour cream, pico de gallo, and cilantro, if desired.