Wonderfully rich, this distinctly nontraditional "pasta" is one of the specialties at Il Sanpietrino in Rome.
FOR PÂTE À CHOUX:
3 tbsp. butter, cut into pieces
Pinch freshly ground black peper
Pinch freshly ground nutmeg
6 tbsp. flour
FOR FILLING AND SAUCE:
1/2 cup dried sliced porcini
2 1/4 cups milk
8 tbsp. butter
3/4 lb. small fresh porcini, quartered
2 cloves garlic, peeled and minced
1/4 cup flour
1/2 cup grated fontina
1/2 cup grated emmentaler
Salt and freshly ground white black pepper
1. For the pâte à choux: Preheat oven to 425°. Combine 1/2 cup water, butter, salt, pepper, and nutmeg in a large heavy saucepan and bring to a boil over medium heat. Remove pan from heat, add flour all at once, and stir vigorously with a wooden spoon until mixture forms a thick dough. Return pan to heat and cook, stirring constantly, until dough pulls away from sides of pan, 1–2 minutes. Remove pan from heat, then vigorously beat in 2 of the eggs, 1 at a time, making sure the first egg is completely incorporated before adding the second. Spoon marble-size pieces of dough 1/2'' apart onto a parchment-lined baking sheet. Lightly beat remaining egg, then brush dough with egg. Bake until puffed and brown, about 20 minutes. Turn off heat, prick each puff with a toothpick, then return them to the oven while making the filling and sauce.
2. For the filling and sauce: Put dried porcini and milk in a medium pot and bring to a simmer over medium heat. Remove from heat, cover, and steep for 30 minutes, then strain and discard porcini. Heat 4 tbsp. of the butter in a large skillet over medium heat and cook fresh porcini and garlic until tender, about 5 minutes; set aside.
3. Preheat broiler. Melt remaining 4 tbsp. butter in a medium-size heavy saucepan over medium-low heat. Add flour and cook, stirring with a wooden spoon, for 2 minutes. Whisk in 2 cups of the warm milk mixture, 1/2 cup at a time, and cook until smooth and slightly thickened, about 4 minutes. Transfer half the sauce to a medium saucepan over low heat, stir in fontina and emmentaler, and season to taste with salt and pepper. Add reserved mushrooms and any juices to other pan, season to taste with salt and pepper, and cook until thickened, about 20 minutes.
4. Slice off tops of each puff. Spoon about 1 tbsp. mushroom filling into each, then cover with tops. Spoon some cheese sauce into the bottom of 6 heatproof dishes. Place 4 puffs in each dish, spoon more sauce over each, sprinkle with parmigiano-reggiano, and broil until lightly browned, about 2 minutes. Garnish with parsley, if you like.