Preheat the oven to 475°F (245°C). If using a pizza stone, place it in the oven to heat.
Prepare the white sauce: In a small saucepan, melt the butter over medium heat.
Add the flour and whisk continuously for about 1 minute until it forms a smooth paste without browning.
Gradually whisk in the warmed milk, continuing to whisk until the sauce thickens and is smooth, about 3-5 minutes.
Remove from heat and stir in salt, black pepper, and garlic powder. Set aside.
Heat olive oil in a skillet over medium heat. Add the sliced mushrooms and sauté until they release moisture and become golden brown, about 5-7 minutes. Remove from heat.
Roll out the pizza dough on a floured surface to a 12-inch circle.
Transfer the dough to a parchment paper sheet or a pizza peel dusted with flour or cornmeal.
Spread the prepared 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.
Sprinkle fresh thyme leaves over the pizza.
Carefully slide the pizza (with parchment, if used) onto the preheated pizza stone or 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 and chopped fresh parsley over the pizza.
Let the pizza rest for 2-3 minutes before slicing and serving.