Drizzle chicken breasts with some olive oil and season with the prepared taco seasoning on both sides (I use about ¾ to 1 teaspoon per chicken breast).
Heat about 1 tablespoon of olive oil in a large skillet over medium heat. Cook the chicken, about 6 minutes per side, or internal temperature registers 165 degrees F. Transfer to a plate and let it rest for 10 minutes. Then, thinly slice the chicken.
On the same skillet, cook bell peppers and onions over medium high heat until tender and slightly charred. Season with salt and pepper to taste. Transfer to a plate and set aside.
To assemble quesadillas: In a clean non-stick skillet, heat ½ tablespoon of butter. Lay a flour tortilla and then a handful of cheese, followed by a few chicken slices and cooked veggies in a single layer (don’t overstuff). Then top with another handful of cheese, and layer over another flour tortilla, pressing down.
Once the bottom tortilla is golden brown and the cheese is melted, remove it from the pan using a wide spatula. Melt another ½ tablespoon butter in the skillet, and cook the other side of the quesadilla until golden brown. Repeat with the remaining tortillas.
Note: You may also arrange the filling on one side of the tortilla and then flip it over with a spatula, if that’s easier.
Slice the quesadillas into wedges. Serve immediately with your favorite toppings.