Heat the oven to 230 C / 450 F with a rack in the middle. Line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment and set the 2 naan on it.
Pat the chicken dry, cut it into 2 cm / 3/4-inch pieces, and toss with the salt, garlic powder, oregano and pepper. Slice the onion and bell pepper thin and shred the mozzarella.
Heat 1 tsp olive oil in a 26 cm / 10-inch cast-iron skillet over medium-high heat until it shimmers. Add the chicken in a single layer and cook 6 to 7 minutes, stirring once or twice, until browned and 165 F / 74 C in the thickest piece. Move it to a plate.
Brush the naan edges with the remaining 2 tsp oil. Spread 60 g sauce on each naan, leaving a 1 cm border, then layer half the mozzarella, the chicken, the onion and pepper, the remaining mozzarella and the Parmesan.
Bake 9 minutes, until the cheese is bubbling with brown spots and the naan edges are deep golden. Cut each pizza into 4 to 6 pieces and add red pepper flakes if you like.