Preheat oven to 425º.
Spray one 8" x 13" sheet pan with cooking spray and place 8 tortillas into the pan. Start by placing 6 tortillas around the perimeter of the pan, overlapping each and allowing them to overhang the side of the pan. Place two additional tortillas down the middle of the pan. Ensure the bottom of the pan is completely covered with the tortillas.
Cook the Knorr Mexican Rice Sides according to directions and add to a large bowl with the cooked chicken, taco seasoning, black beans, corn, bell pepper and onion then mix to combine.
Spread the filling over the prepared tortillas.
Place two additional tortillas atop the filling, down the middle of the pan. Then fold the overhanging tortillas over the filling, overlapping to completely cover the filling.
Brush the tops of the tortillas with melted butter and place a second sheet on top of the tortillas.
Bake at 425º for 20 minutes. Remove the top pan and bake for another 8 - 10 minutes, until the top is golden and crispy.
Remove from oven, cool slightly then slice and serve with your preferred toppings.