In a medium saucepan, sauté porcinis in 6 tablespoons butter until soft; set aside in a bowl.
In the same pan, soften shallot in remaining 2 tablespoons butter over medium-low heat; add wine, increase heat and bring to a simmer, reducing volume by two-thirds, 7 to 8 minutes.
Add stocks; simmer on medium-low heat, 15 minutes, skimming as needed.
Add thyme and bay leaf; cook 1 minute. Add reserved mushrooms. Season with vinegar, salt and pepper. Reserve.
Melt 2 tablespoons butter in small sauté pan; add panko crumbs and cook until lightly toasted, 1 to 2 minutes. Toss with Parmigiano-Reggiano; reserve.
Butter earthenware or glass baking dish (1 3/4 inches to 2 1/2 inches deep, 16-ounce capacity) using remaining 2 teaspoons butter.
Slice potato into 1/8-inch-thick slices. Combine with cream, cheese, garlic and nutmeg in large mixing bowl. Season with salt and pepper.
Slice 3/4 of the truffle paper-thin.
Layer potatoes, spreading cheese around potato slices. Add a thin layer of truffles; top with layers of remaining potatoes and truffles. (Do not mix in truffles.)
Cover dish with aluminum foil, place on sheet pan; bake at 325°F until just fork tender, 30 to 50 minutes. Remove foil; top with reserved Parmigiano-Reggiano/breadcrumb mixture. Place in a 550°F oven to brown the top, 3 to 4 minutes. Remove bay leaf and thyme.
Slice remaining 1/4 of truffle paper thin.
Garnish gratin with chopped parsley and truffle slices.
Per portion: Place 3 to 4 ounces of gratin onto serving plate; spoon 1 1/2 to 2 ounces sauce around gratin.