How to Make It
In a large, deep skillet, melt the butter. Add the leeks and garlic and cook over moderately low heat until softened, about 10 minutes. Add the mushrooms and cook, stirring occasionally, until tender and any exuded liquid has evaporated, about 20 minutes. Stir in the rosemary and season with salt and pepper; keep warm.
Meanwhile, in a small saucepan, bring the heavy cream to a boil. Cook over moderate heat until reduced to 2/3 cup, about 7 minutes. Remove the saucepan from the heat and stir in the Parmigiano-Reggiano and pecorino until melted. Strain the fonduta into a medium bowl.
Spoon the sautéed mushrooms and leeks onto a platter and drizzle with the vin cotto. Spoon the pecorino fonduta on top and serve immediately with the grilled Italian bread.
Vin cotto, a sweet, tangy, grape must condiment, is available at specialty-food stores and online at igourmet.com.