In a medium saucepan over low heat, add the milk and 1/2 cup of the water and warm to about 100 degrees F.
Transfer the saucepan from the heat and sprinkle the yeast over the milk mixture.
Sprinkle 1/8 teaspoon of the sugar over the top of the yeast mixture and set it aside, without stirring, to bloom until foamy, about 5 minutes.
Whisk in 4 tablespoons of the melted butter and the egg yolk into the yeast mixture.
In a large bowl, whisk 2 3/4 cups of the flour, the remaining sugar, and 1 teaspoon of the salt together.
Make a well in the center of the flour mixture and stir in the yeast mixture with a wooden spoon until a thick and slightly sticky dough forms.
Lightly dust a work surface with the extra flour.
Turn the dough out onto the floured work surface and knead until it is soft and elastic, about 6 minutes.
Shape the dough into a ball.
Brush the inside of a large bowl with the extra melted butter.
Add the dough ball to the greased bowl and turn it to coat lightly.
Cover the bowl with plastic wrap and note the time.
Let the dough rise in a warm place until the dough doubles in size, about 1 hour 30 minutes.
Turn the dough out of the bowl and knead it briefly to release the excess air.
Reform the dough into a ball and return it to the greased bowl.
Lightly grease a large piece of plastic wrap with the extra melted butter and lay it on the dough.
Cover the entire bowl tightly with the greased plastic wrap and refrigerate for at least 4 hours and up to overnight to let the dough proof.
Spread the poppy seeds, the sesame seeds, the garlic flakes, and the onion flakes on a pie plate.
Toast the seasoning mixture in the oven until lightly browned and fragrant, about 3 minutes.
Let the toasted seasoning mixture cool for 5 minutes.
In a small bowl, mix in the cream cheese, the remaining whole egg, and the remaining 1/4 cup of the flour until combined.
Stir in 5 of the sliced scallions, the lemon zest, the salt, and the pepper.
Butter a 9x13-inch baking pan with 2 tablespoons of the room temperature butter.
Dust a work surface using the extra flour.
Turn the dough onto the floured work surface and press it flat.
Roll the dough into a 10x18-inch rectangle and lay it vertically.
Spread the filling mixture evenly over the dough, leaving a 1-inch border on the top portion of the dough.
Reserve 2 tablespoons of the toasted seasoning mixture and then scatter the remaining seasoning mixture over the filling.
Starting from the bottom, roll the dough up into a tight roll.
Lightly brush the 1-inch border with the extra water and continue to roll to seal.
Fold the ends and even out the rolls using your hands.
Cut the rolls every 1 1/2 inches with a serrated knife to make 12 buns.
Place the buns cut-side down in the prepared pan leaving 1 inch of space between the rolls.
Cover the buns loosely with plastic wrap and set the pan in a warm place to rise until they double in size with no gaps in between, 1 hour 30 minutes-2 hours.
Position an oven rack in the center of the oven and preheat the oven to 350 degrees F.
Bake the buns until they are golden-brown and the tops spring back when pressed lightly, about 35-40 minutes.
Stir the remaining 4 tablespoons of the butter, the reserved seasoning mixture, the remaining scallions, and the salt together.
Spread the topping mixture evenly over the buns and return the buns to the oven until the butter is melted and the topping is fragrant, about 5 minutes.
Let the buns cool in the pan for 10 minutes.
Serve.