If using store-bought dough, let it come to room temperature for at least 30 minutes before stretching. Preheat your oven to 475°F (245°C) with a pizza stone or baking sheet inside for at least 30 minutes prior to baking.

In a medium saucepan, melt butter over medium heat. Add minced garlic and sauté for 30 seconds until fragrant, do not brown. Whisk in the flour and cook for 1 minute, stirring constantly to form a roux. Gradually whisk in the warm milk, stirring continuously to prevent lumps. Bring to a simmer, then reduce heat to low and cook for 2-3 minutes until thickened. Stir in salt, pepper, Parmesan cheese, and optional nutmeg. Remove from heat and set aside.

Heat olive oil in a large skillet over medium-high heat. Add thinly sliced mushrooms and cook, stirring occasionally, for 5-7 minutes until they release their liquid and begin to brown. Add minced garlic, fresh thyme, salt, and pepper. Cook for another 1-2 minutes until garlic is fragrant. Remove from heat.

On a lightly floured surface or a piece of parchment paper, gently stretch or roll out the pizza dough into a 12-14 inch circle. If using a pizza peel, lightly dust it with cornmeal or semolina, then carefully transfer the dough to the peel.

Spread the prepared white sauce evenly over the dough, leaving a 1/2-inch border for the crust. Distribute the sautéed mushrooms over the sauce. Sprinkle both mozzarella and Fontina cheeses evenly over the mushrooms.

Carefully slide the pizza (with parchment or directly from the peel) onto the preheated pizza stone or baking sheet in the oven. Bake for 12-15 minutes, or until the crust is golden brown and the cheese is melted and bubbly with some browned spots.

Carefully remove the pizza from the oven. Let it rest for 2-3 minutes before slicing. Garnish with fresh parsley or chives and a sprinkle of red pepper flakes, if desired. Serve hot.