Pat the chicken dry, rub it with a little olive oil, and sprinkle the taco seasoning evenly over both sides. Use about 3/4 to 1 teaspoon of seasoning per breast.

Heat about 1 tablespoon of the olive oil in a large skillet over medium heat. Cook the chicken for about 6 minutes per side, or until an instant-read thermometer registers 165°F. Move the chicken to a plate and let it rest for 10 minutes, then slice it thinly.

Return the skillet to medium-high heat. Add the bell peppers and onion and cook until tender and lightly charred, about 5 to 7 minutes. Season with salt and pepper, then transfer the vegetables to a plate.

Assemble the quesadillas in a clean nonstick skillet set over medium heat. Add 1/2 tablespoon of the butter, then lay one tortilla in the pan. Scatter a handful of cheese over the tortilla, add a few slices of chicken and some vegetables in a single layer, and do not overstuff. Top with another handful of cheese and place a second tortilla on top, pressing down gently.

Cook until the bottom tortilla is golden brown and the cheese has melted. Use a wide spatula to lift the quesadilla out of the pan. Melt another 1/2 tablespoon of butter in the skillet, return the quesadilla, and cook the second side until it is golden brown. Repeat with the remaining tortillas, butter, chicken, vegetables, and cheese.

For an easier option, arrange the filling on just one half of the tortilla, fold the other half over, and flip the quesadilla once with the spatula.

Slice the quesadillas into wedges and serve right away with your favorite toppings.