First, make the sauce. Melt the butter in a small saucepan over medium heat. Add the flour and whisk to blend and cook the flour, about 1 minute. Add the garlic.
Add the milk and whisk constantly until smooth and the flour lumps are gone. Add the salt and pepper and stir in the parmesan cheese. Whisk for thirty seconds and remove from heat. Transfer to a small bowl and set aside to cool slightly before using (you can also make the sauce ahead of time).
Preheat the oven to 425F. If you have a pizza stone, place in the oven to preheat.
On a flour dusted surface, roll out the pizza dough into a 12" round pizza. Place on the pizza stone or a large cookie sheet. Poke the center of the dough with a fork all the way around, leaving room for the crust.
Bake the crust for 7 minutes to allow it to partially cook.
Remove the crust from the oven and spread the cooled sauce over it. Sprinkle with the mozzarella cheese. Distribute the mushroom around the pizza evenly, followed by the fresh rosemary.
Bake the pizza for 12-15 minutes until the crust has cooked through and the pizza is golden brown. Remove from the oven and allow to cool for five minutes before cutting into 8 pieces.