Preheat oven 400 F / 200 C and prepare a cast iron skillet or baking tray by lightly greasing it.
For the BBQ Chicken:
In a medium sized bowl, combine the chicken and BBQ sauce and mix. Set aside.
For the Nachos:
Spread the Doritos onto the cast iron skillet in one layer.
Spread the chicken, green bell pepper, onion and tomato evenly onto the Doritos.
Sprinkle over the cheddar and mozzarella cheese so that it evenly covers the entire surface.
Bake for about 5 minutes until heated through and the cheese has melted.
You may top the Doritos with additional diced bell pepper, onion and tomato if desired.
Serve immediately with guacamole, tomato salsa, and sour cream if desired.