Step 1Cut off the dark green tops of the leeks (discard or save them for another use), saving the white and light green parts. Halve the leeks lengthwise, leaving them attached near the base, and rinse them thoroughly, spreading the layers to rinse off any sand between them. Cut the leek halves in crosswise slices about 1/4 inch thick. Submerge the sliced leeks in a bowl of water and rub to separate the pieces. Set them aside for about 2 minutes, and then drain them in a colander. If the water is sandy, soak them and drain again.
Step 2Finely mince the parsley in a food processor or with a knife. Add the garlic and mince together.
Step 3In a large skillet, heat half of the butter and oil over medium-high heat until the butter melts. Add the mushrooms, sprinkle them with salt and pepper and cook, without stirring, until they begin to brown, 2 to 3 minutes. Stir, raise the heat to high and continue to sauté until they are lightly browned and any liquid has evaporated, 4 or 5 minutes more. Remove from heat and transfer them to a plate.
Step 4Add the remaining olive oil and butter to the skillet and heat over medium-low heat. Add the leeks and season with 1/4 teaspoon salt and several grinds of pepper, or to taste. Cover and cook, stirring occasionally, until the leeks are very tender, about 18 to 20 minutes. If there is liquid in the pan, uncover the pan and continue to cook over medium heat, stirring occasionally, until the liquid evaporates, about 3 minutes.
Step 5If desired, add the cream to the leeks and cook, stirring, until the cream is absorbed, about 4 minutes.
Step 6Add the mushrooms to the leeks and heat through. Stir in the parsley and garlic mixture. Taste, adding an additional ¼ teaspoon salt and several grinds of pepper, along with 1/8 teaspoon cayenne pepper, or to taste. Remove from heat. This makes about 2 cups filling. If not using right away, set aside to cool, then cover and refrigerate until ready to fill the gougeres, up to 2 days. Reheat before filling.
Step 1Position a rack in the center or just below the center of the oven and preheat to 400 degrees. Grease the corners of two baking sheets and line them with parchment paper; or line them with foil and grease the foil.
Step 2In a small saucepan, combine the water, butter, olive oil and salt over medium-low heat, cooking until the butter melts. Increase the heat and bring the mixture to a boil. Remove from heat and add the matzo cake meal all at once. Mix well. Return the pan to low heat and cook, stirring constantly, until the mixture is a thick paste, about 1 minute. Remove from heat and transfer the mixture to the bowl of a stand mixer or another large bowl and set aside for 5 minutes.
Step 3Beat one egg into the mixture until completely incorporated, using the paddle attachment (if using a stand mixer) or a wooden spoon (by hand). Continue to beat in the eggs, one at a time, until fully combined. Add the black pepper and cayenne and beat until incorporated and the mixture is smooth. Finally, slowly beat in the cheese.
Step 4Take rounded tablespoons (about 1 1/2 measuring tablespoons each) of dough and place the dough onto baking sheets, spacing each rounded mound about 1 ½ inches apart. If there are points on the mounds, moisten your finger with water and push them down.
Step 5Place the baking sheets on the rack in the center of the oven. (If 2 baking sheets don’t fit on the same rack, reserve the second baking sheet at room temperature.) Bake for 15 minutes.
Step 6Reduce the oven temperature to 375 degrees and bake the puffs until golden and firm, 15 to 20 more minutes.
Step 7Turn off oven and wedge door slightly open with wooden spoon. Leave puffs in oven for 30 minutes to give them time to dry out.
Step 8Remove the puffs from the oven. Using the point of a paring knife, pierce each puff at the center of one side to allow steam to escape.
Step 9If you have a second batch of puffs to bake, reheat oven to 400 degrees. Bake the second batch of puffs as above.
Step 10Just before serving, rewarm the leek and mushroom filling. Taste the filling, and adjust for seasoning if needed. Using a serrated knife, carefully cut off the top half of each puff and reserve this part as a “hat.” Keep each puff “hat” with its base.
Step 11Fill the puffs, and serve warm or at room temperature.
Step 12Like other cream puff shells, those made for Passover are best on the day they are baked. You can store unfilled shells for a day in an airtight container at room temperature. If you’re keeping the shells for longer than that, it’s best to freeze them; however you keep them, it’s worth heating them in the oven or toaster oven at medium or medium-low heat to restore their freshness. After you have filled the cream puff shells, you can keep them in a closed container in the refrigerator for up to a day.