Cut leeks lengthwise and rinse thoroughly under cold water. Then cut the halved leeks into 1-inch half-moons.
Place leeks, thyme, and garlic in the bottom of a metal steamer. Season with salt and pepper. Place the basket over simmering water and cover until leeks are tender, about 10 minutes. Discard the garlic and thyme. Remove the leeks and set aside.
In a separate bowl, mix the lemon zest, breadcrumbs and parsley. Add breadcrumb mixture to the leeks, then add the Champagne vinegar and drizzle with extra virgin olive oil. Season with salt and pepper to taste. Add the mushrooms and gently toss the leeks to combine all ingredients then place on a plate or serving plater. Finish with grated Castelmango cheese and enjoy.