Melt the butter in a small saucepan over medium heat.
Whisk in the flour until smooth and cook, stirring, for about 2 minutes to form a roux.
Gradually whisk in the milk and cook until the sauce is slightly thickened, a few minutes more.
Stir in the minced garlic, chopped basil, salt and pepper to taste, then mix in the grated Parmesan until melted; remove from heat.
Preheat the oven to 475°F (245°C). If using homemade dough, prebake the stretched pizza crust for 5 minutes on a baking sheet or pizza stone.
Remove the partially baked crust and spread an even layer of the white garlic sauce over it.
Top with shredded mozzarella, diced cooked chicken, diced tomato, sliced mushrooms, chopped onion, chopped artichoke hearts, and fresh spinach; sprinkle a little more mozzarella on top if desired.
Bake at 475°F (245°C) for 10–12 minutes more, until the crust is golden and the cheese is melted and bubbly.
Remove from oven, let rest a few minutes, then slice and serve.