Step 1

Preheat the oven to 400 degrees F (200 degrees C). Line a large baking sheet with foil and spray lightly with cooking spray.

Step 2

Melt butter for sauce in a small saucepan over medium heat. Add garlic and saute until fragrant, about 30 seconds. Stir in flour and whisk for 1 to 2 minutes. Pour in milk and whisk until sauce starts to thicken, 3 to 4 minutes. Stir in Parmesan cheese, salt, and pepper; cook until cheese is melted, about 1 minute. Set aside to cool, about 5 minutes.

Step 3

Spread cooled sauce over the flatbread and sprinkle with mozzarella cheese. Top with spinach, chicken, tomatoes, onion, and salt. Drizzle with olive oil and sprinkle with Parmesan cheese. Transfer to the prepared baking sheet.

Step 4

Bake in the preheated oven until cheese is melted and crust is lightly browned, 12 to 15 minutes. Cool slightly before serving.