Preheat the oven to 475°F (245°C). If you have a pizza stone, place it in the oven to heat.
In a medium saucepan, melt the butter over medium heat.
Whisk in the flour and cook for 1-2 minutes until it forms a smooth paste and begins to bubble, but do not brown.
Gradually whisk in the warmed milk, continuing to whisk until the sauce thickens, about 3-5 minutes.
Remove the sauce from heat and stir in salt, black pepper, and garlic powder. Set aside.
In a skillet, heat olive oil over medium heat. Add the sliced mushrooms and sauté until tender and lightly browned, about 5 minutes. Stir in fresh thyme leaves and remove from heat.
Roll out the pizza dough on a floured surface to about 12 inches in diameter.
Transfer the dough to a pizza peel or baking sheet lined with parchment paper.
Spread the white sauce evenly over the pizza dough, leaving a small border around the edges.
Sprinkle the shredded mozzarella and fontina cheeses evenly over the sauce.
Distribute the sautéed mushrooms evenly on top of the cheese.
If using a pizza stone, carefully slide the pizza (with parchment) onto the preheated stone. Otherwise, place the baking sheet in the oven.
Bake for 12-15 minutes until the crust is golden and the cheese is bubbly and slightly browned.
Remove the pizza from the oven and immediately drizzle the truffle oil evenly over the top.
Sprinkle grated Parmesan cheese over the pizza.
Let the pizza rest for 2-3 minutes before slicing and serving.