In a medium skillet, place 2 tbsp olive oil and chicken breast. Sprinkle chicken with a thin layer of taco seasoning, turn, and sprinkle the other side. Cook chicken completely through. Set aside.
Meanwhile, in a large skillet, saute peppers and onion in 2 tbsp olive oil until onions are golden and peppers are soft, about 15-20 minutes. (Optional: sprinkle vegetables with taco seasoning powder.) Remove vegetables from skillet and wipe down the skillet.
Shred the chicken in small pieces. Reheat the chicken in the medium skillet, adding taco sauce and stirring to mix thoroughly.
Spray the large skillet with non-stick cooking spray. Place 1 flour tortilla in skillet for 1-2 minutes until it is just beginning to brown. Set aside. Place another tortilla in the skillet. Layer shredded cheese, chicken in sauce, peppers and onions, and baby spinach leaves. Top with additional cheese and replace the first (toasted) tortilla on top.
Cook the quesadilla, pressing down with a spatula, until bottom tortilla is a bit brown and crispy and cheese is melted.
Repeat the previous two steps to make a second quesadilla.
Cut each quesadilla into quarters and serve with sour cream, salsa, or guacamole on the side.