- Preheat the oven to 450ºF (230ºC) and arrange the rack to the lowest position. Line a large baking sheet with parchment paper; set aside. Stretch the dough out, on a lightly floured surface, into an 18 x 12-inch (45 cm x 30 cm) rectangle. Transfer to the prepared pan.
- Stir the sauce with the spinach and spread over the dough, leaving a ½-inch (2 cm) border all the way around. Scatter chicken, artichokes, red pepper and red onion over the sauce. Sprinkle mozzarella and Parmesan over the toppings.
- Bake for 18 to 20 minutes or until the bottom is golden and cheese is melted. Let stand for 5 minutes before cutting into pieces.
Broil the flatbread for 3 to 5 minutes to brown the cheese.