How to Make It
Place a cast-iron pizza pan or pizza stone in oven, and preheat oven to 550°F. (Do not remove pizza pan while oven preheats.) Heat 2 tablespoons of the oil in a large skillet over medium-high. Add mushrooms, leek, thyme, pepper, and 1 1/4 teaspoons of the salt; cook, stirring occasionally, until vegetables are tender, about 6 minutes. Remove from heat.
Sprinkle a pizza peel with cornmeal; top with 1 pizza dough round. Brush dough with 1 tablespoon of the oil, and sprinkle with 1/4 teaspoon of the salt. Top dough with half each of mozzarella, fontina, Parmigiano-Reggiano, Gorgonzola, and mushroom mixture. Slide pizza onto preheated pizza pan. Bake until golden brown and charred in spots, 8 to 10 minutes. Let stand 5 minutes; transfer to a cutting board. Garnish with thyme leaves, if desired. Wipe pizza pan clean, and repeat with cornmeal and remaining dough round, oil, salt, and toppings.