Preheat the oven to 350°F (175°C). Lightly butter a 9 x 13-inch (23 x 33-cm) baking dish. Thaw the phyllo according to its package directions, and keep unused sheets loosely covered with a barely damp towel.

Lay one phyllo sheet on the counter with a short side facing you. Gently gather it back and forth into loose 1-inch accordion folds. Place the pleated strip in the dish with the folds facing upward. Repeat with all 16 sheets, arranging them side by side without tightly packing them.

Bake the folded phyllo for 10 minutes to dry the surface slightly. Remove the dish, drizzle the 1/2 cup melted butter evenly into and over the folds, and return it to the oven for 10 minutes.

While the pastry prebakes, whisk the eggs and sugar together. Add the milk, cream, finely ground poppy seeds, vanilla, lemon zest, and salt, whisking until evenly combined.

Remove the dish from the oven. Slowly pour the custard over the pastry, pausing so it can flow into the gaps rather than collecting only on top. Do not press down or flatten the folds.

Bake for 30 to 35 minutes more, until the ridges are deep golden and the custard in the center is set with only a slight wobble. If the top browns too quickly, lay foil loosely over the dish for the last few minutes.

Warm the honey, lemon juice, and water together just until fluid. Drizzle lightly over the hot pie, concentrating on the center and leaving some crisp ridges exposed.

Rest for 15 to 20 minutes before cutting into 12 squares. Dust with powdered sugar, if using, just before serving.