For gnocchi: Preheat oven to 450F. Pierce potatoes with fork. Bake until tender, about 1 hour. Cool slightly; cut potatoes in half and scoop pulp into large bowl. Mash potatoes well. Mix in egg yolk, salt, pepper and nutmeg. Mix in enough flour to form firm, slightly elastic dough. Turn out onto lightly floured surface. Divide dough into 4 equal portions. Gently roll 1 portion between hands and work surface to form 1/2-inch-thick rope about 18 inches long. Cut crosswise into 1/2-inch pieces. Roll each piece over tines of dinner fork to make grooves in gnocchi. Arrange gnocchi in single layer on floured baking sheet. Repeat with remaining 3 portions of dough. Working in small batches, cook gnocchi in large pot of boiling salted water until gnocchi rise to top and are tender, about 5 minutes. Using slotted spoon, transfer gnocchi to large baking pan. (Gnocchi can be prepared ahead. Let stand 1 hour at room temperature, or cover and refrigerate up to 2 days.) For mushroom sauce: Cook butter and olive oil in heavy large skillet over medium heat until butter begins to brown, about 2 minutes. Add mushrooms and shallots and saute until golden brown, about 10 minutes. Add stock and sage; simmer until liquid is slightly reduced, about 8 minutes. Season to taste with salt and pepper. Add gnocchi to sauce in skillet and cook until heated through, about 1 minute. Add arugula and stir until wilted. Divide equally among bowls. Drizzle with truffle oil, if desired. Sprinkle with cheese. *Available at Italian markets, specialty foods stores and some supermarkets. Makes 4 Servings. Bon Appetit November 1999 Per serving: 1164 Calories (kcal); 64g Total Fat; (48% calories from fat); 28g Protein; 124g Carbohydrate; 296mg Cholesterol; 1845mg Sodium Food Exchanges: 8 Grain(Starch); 2 Lean Meat; 0 Vegetable; 0 Fruit; 11 1/2 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.