Preheat oven to 450°.
Heat a 12-inch nonstick skillet over medium-high heat. Coat pan with cooking spray. Add mushrooms and shallots to pan; sauté 7 minutes or until mushrooms are tender. Add garlic and thyme; sauté 1 minute. Stir in vinegar; remove from heat.
Place crust on the bottom rack of oven. Bake at 450° for 4 minutes.
Place the crust on a baking sheet. Spread mushroom mixture evenly over crust; sprinkle evenly with prosciutto and fontina cheese. Bake at 450° for 6 minutes or until cheese melts.
Tips: For a crisp crust, bake the pizza crust on the lowest rack for a few minutes before adding the toppings. Presliced button mushrooms can substitute for the creminis.