1. To make puffs, preheat oven to 300 degrees Fahrenheit (150 degrees Celsius).
2. Combine water, salt, and butter in a small saucepan and cook over medium flame until butter is melted and mixture is boiling. Stir and add cornstarch and flour simultaneously. Continue stirring as the mixture cooks. Remove from flame as soon as a dough forms and separates from the sides of the pan.
3. Transfer hot dough to mixer or food processor. Using beater or knife blade on medium power, add eggs, one at a time, until completely mixed into dough. Transfer dough into a piping bag and form approximately 2 and ½-inch-wide (6 cm) pastry circles on Gefen Parchment-lined baking pan. Leave space between the circles to allow them to rise.
4. Bake puffs for approximately 15 minutes. They should be high and golden. (If necessary, move to a higher rack in the oven and bake for another five minutes until golden.) Turn off oven and open door slightly. Place a towel over door, making sure to allow air to enter, as the puffs cool.
5. Combine all ingredients for cheese filling in a bowl. Refrigerate and fill puffs no more than an hour before serving.
6. Meanwhile, heat oil in a frying pan over low flame and sauté onions and mushrooms for approximately 20 minutes. Add milk, garlic, salt, and nutmeg and bring to boil. (Stir carefully to ensure that milk doesn’t boil over.) Lower flame and cook approximately 10 more minutes. Transfer to food processor or blender (you can wait until the sauce cools) and add sour cream. Blend until sauce is creamy.
7. To assemble, open cooled dough puffs slightly and fill with a teaspoon of filling. Pour hot cream sauce over serving platter. Lay filled cheese puffs on top and drizzle olive oil all around. Garnish with parsley leaves and serve immediately. Place remaining cream sauce in a bowl in the center of the table.
The unfilled puffs can be stored in a sealed container for two days, or frozen. The filling lasts for one week in the refrigerator. Sauce can be stored for three days in refrigerator or frozen. Reheat thoroughly before assembly.