Preheat the oven to 425°F.
Spray one of two 18x13-inch baking sheets with nonstick cooking spray.
Combine fajita seasoning with 3 tablespoons of water in a small bowl and stir well.
Heat 1 ½ tablespoons of vegetable oil in a skillet over medium-high heat.
Sauté the sliced onions for 2 to 3 minutes until they start to soften and turn opaque.
Add sliced green bell peppers and cook for another 2 to 3 minutes until vegetables are tender, then transfer to a plate.
Add the remaining 1 ½ tablespoons oil to the skillet and heat.
Cook the sliced chicken breast for 5 to 6 minutes until no longer pink, stirring frequently.
Pour the fajita seasoning mixture over the chicken and cook for 2 to 3 minutes until fully coated and seasoned, then transfer to a bowl.
Arrange 6 flour tortillas around the edges of the sprayed baking sheet, overlapping and hanging halfway over the edges, leaving the center open.
Place one tortilla over the center gap at the bottom.
Cut the last tortilla in half using a sharp knife or pizza cutter.
Sprinkle 2 cups of shredded cheese evenly over all tortillas.
Evenly spread the sautéed vegetables over the cheese layer.
Distribute the cooked chicken strips over the vegetables.
Spread the rinsed and drained black beans on top of the chicken.
Top with the remaining 2 cups shredded cheese and sprinkle chopped cilantro over the cheese.
Starting at either side, carefully fold the tortillas towards the center, pressing edges together; dampen edges with water if needed to help seal.
Dampen edges of the halved tortillas and lay each half over the center; press to adhere.
Place the second baking sheet on top to press down and keep edges from separating; optionally weigh down with pie weights.
Bake for 20 minutes, remove the top sheet pan, and bake another 5 to 7 minutes until golden brown.
Let the quesadilla rest 2 to 3 minutes before cutting into 3 by 4 slices.
Serve hot with optional garnishes like sour cream, guacamole, or pico de gallo.