Preheat oven to 500 degrees F with pizza stone, if using. Sprinkle cornmeal over baker's peel or parchment.
Roll pizza dough into a 14 inch circle, place on prepared baker's peel, and brush the edge with olive oil.
Add butter to a small saucepan and melt over medium heat. Once melted, add the garlic and cook until just fragrant, 30 seconds to 1 minute. Whisk in the flour and pepper and cook until golden. Continue whisking and slowly pour in the milk. Bring to a simmer and cook, stirring often, until thickened. Remove from heat and stir in Parmesan, salt, and pepper.
Spread about 1/2 cup sauce over prepared pizza dough, enough for a thin layer. Sprinkle chicken and onions over the pizza. Cover with mozzarella.
Transfer the pizza to pizza stone using baker's peel or parchment. Bake until crust is golden and cheese is bubbly, 10-12 minutes. Remove from oven and let cool about 5 minutes before topping with fresh herbs and slicing. Serve with remaining white sauce.