Cook the lobster with the Chablis in a large pan, covered, until the lobster is cooked through and the flesh is white throughout, about 10 minutes. Remove the lobster from the pan.
Clean and dice the lobster and set aside. Place the shells back in the pan and add 3 cups water. Cook the lobster stock for 1 hour. Strain the stock into a saucepan, then reduce the lobster stock a bit.
Add butter to a large skillet over medium-high heat, then add the mushrooms with some salt and pepper and cook until the mushrooms start to soften and brown, about 10 minutes. Add the reduced lobster stock, heavy cream, half the truffle, cooked lobster, lemon juice and some salt and pepper. Heat until combined and set aside.
Preheat the broiler. Cut the baguette in half lengthwise, scoop out the flesh inside, then butter lavishly. Toast the baguette in the broiler until golden brown, about 5 minutes.
Cut the baguette into thick slices and arrange on a board. Fill a serving bowl with the fonduta. Garnish with the remaining half truffle.
This recipe has been updated and may differ from what was originally published or broadcast.