2 1/4 cups
                        all-purpose flour, plus more for rolling out the dough
                                        
                        
                        1 teaspoon
                        baking powder
                                        
                        
                        1/2 teaspoon
                        kosher salt
                                        
                        
                        1
                        large egg, lightly beaten
                                        
                        
                        8 tablespoons
                        unsalted butter, melted and cooled
                                        
                        
                        1 teaspoon
                        distilled white vinegar
                                        
                        
                        1/2 cup
                        lukewarm water
                                        
                        
                        4 tablespoons
                        unsalted butter
                                        
                        
                        2
                        large white or yellow onions, thinly sliced
                                        
                        
                        1 teaspoon
                        kosher salt
                                        
                        
                        1/2 teaspoon
                        freshly ground black pepper
                                        
                        
                        1 teaspoon
                        granulated sugar
                                        
                        
                        2
                        medium leeks, whites only, trimmed, cleaned, and thinly sliced
                                        
                        
                        1/4 cup
                        water, plus one teaspoon
                                        
                        
                        1 pound
                        Yukon gold potatoes, peeled
                                        
                        
                        6 ounces
                        all-natural cream cheese
                                        
                        
                        1 bunch
                        fresh chives, minced (about 1/4 cup)
                                        
                        
                        1/4 cup
                        Parmigiano-Reggiano cheese, finely grated (optional)
                                        
                        
                        1 1/2 teaspoons
                        white or black sesame seeds
                                        
                        
                        1 teaspoon
                        poppy seeds
                                        
                        
                        1 teaspoon
                        caraway seeds
                                        
                        
                        1 tablespoon
                        dried onion flakes
                                        
                        
                        1
                        large egg, beaten