Mix cumin, chili powder, smoked paprika, garlic powder, salt, and black pepper in a small bowl. Pat the chicken dry with paper towels and coat it evenly all over with the spice mix.
Heat 1 tbsp olive oil in a large skillet over medium-high heat until it shimmers. Add the chicken and cook for 6 to 7 minutes on the first side undisturbed, until golden. Flip and cook for 5 to 6 more minutes until the internal temperature reaches 75°C (165°F) and the juices run clear.
Remove the chicken from the pan and rest on a cutting board for 5 minutes. Shred into strips with two forks or slice thinly.
Return the skillet to medium heat (no additional oil). Add the sliced bell peppers and red onion and cook for 4 to 5 minutes, stirring occasionally, until softened with slightly charred edges and all moisture has evaporated.
Add the shredded chicken back into the skillet. Stir to combine and cook for 1 minute. Remove from heat.
Lay one flour tortilla flat. Cover one half with about 50g of shredded cheese, then a quarter of the chicken filling, then a thin layer of additional cheese. Fold the empty half over.
Heat a large, dry skillet (no oil) over medium heat for 1 to 2 minutes. Place the quesadilla in the pan and cook for 2 to 3 minutes until the underside is deep golden and feels firm under the spatula.
Flip in one smooth motion and cook for a further 2 to 3 minutes until golden and the cheese is fully melted with no cold spots.
Rest on a cutting board for 1 minute, then cut into 3 to 4 wedges. Repeat with remaining tortillas and filling.
