Preheat oven to 375°F (190°C). Spray a baking sheet with cooking spray. If your chicken is not cooked, halve the chicken breasts, place them on the baking sheet, and sprinkle with 1/2 tsp garlic salt.
Place 6 garlic cloves on a piece of foil folded into a small raised tray so juices won’t reach the garlic, and put the foil on the same baking sheet. Roast the chicken and garlic together until the chicken reaches 165°F (74°C) and juices run clear. Remove from oven, peel and mince the roasted garlic, and chop the chicken into bite-sized pieces.
Increase oven temperature to 400°F (200°C). Spray a pizza pan or baking sheet with cooking spray and stretch the refrigerated pizza crust to desired thickness. Bake the crust for about 5 minutes until just set, then remove from oven.
While the crust is par-baking, make the white sauce: melt 1 tbsp butter in a medium saucepan over medium heat. Add the roasted minced garlic and cook about 2 minutes, stirring.
Add 1/2 cup milk and 1/4 cup half & half to the pan and whisk. Reduce heat to medium-low and sprinkle 2 tbsp flour over the liquid, whisking constantly to prevent lumps.
Stir in 1/2 tsp ground black pepper, 1/4 tsp dried thyme, 1/2 tsp dried oregano, and 1/2 tsp garlic salt; whisk until smooth. Remove from heat and stir in 1/4 cup grated Parmesan until melted and combined.
Spoon the white sauce evenly over the half-baked pizza crust. Top with the chopped cooked chicken, then sprinkle 3/4 cup shredded mozzarella evenly over the pizza, and scatter 1/4 cup chopped red onion on top.
Bake the assembled pizza at 400°F (200°C) for 5–6 minutes. Turn on the broiler and broil 1–3 minutes more until cheese and crust are browned to your liking. Remove, let rest briefly, slice, and serve.