1. 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.
2. 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.
3. 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.