Provided by: Chef Philippe Bertineau
Heat the oil in a sauté pan, add the shiitakes, and sweat over low heat. Add 2 garlic cloves, 1 sprig thyme, and 1 cup of the stock; bring to a boil; and reduce the liquid until dry. Season to taste with salt and pepper. Set aside. (May be made a day ahead and refrigerated.)
Melt 1 teaspoon butter in a sauté pan. Add the celery, shallots, 1 sprig thyme, and 2 garlic cloves, and sweat over low heat for 3 minutes. Add chestnuts, and cook for 5 more minutes; add 2 cups stock and simmer until liquid has evaporated. Season with salt and pepper, and remove the garlic and thyme. Add the remaining butter, and crush the chestnuts with a fork until the butter is incorporated. Set aside. (May be made a day ahead.)
Sweat the onion, ginger, and lemongrass in a saucepan till the onion is soft. Add the diced apple, and cook for 3 minutes. Add the rice vinegar and syrup, reduce until dry. Season to taste with salt and pepper and remove the ginger and lemongrass. (May be made a day ahead.)
Spoon a little chestnut purée into each mushroom cap, and top with the apple chutney. (May be assembled a few hours ahead.)
Preheat the oven to 350 degrees. Set the mushroom caps on a baking sheet, and heat them in the oven for 5 minutes. Garnish each with a sprig of chervil and place in a porcelain Chinese spoon.