1. For the dough: Warm the milk and 1/2 cup water in a medium saucepan over low heat to about 38°C. Remove the saucepan from the heat, and sprinkle the yeast over the liquid. Sprinkle a pinch of the sugar over the top, and set aside without stirring until foamy, about 5 minutes. Then whisk in the butter and egg yolk.
2. Whisk together the flour, remaining sugar and salt in a large bowl. Make a well in the center, and stir in the yeast mixture with a wooden spoon until you have a thick, slightly sticky dough. Turn the dough onto a floured work surface, and knead until soft and elastic, about 6 minutes. Shape the dough into a ball.
3. Brush the inside of a large bowl with butter. Add the dough ball, turning to coat lightly. Cover the bowl with plastic wrap, trace a circle the size of the dough on the plastic and note the time. Let the dough rise in a warm place until doubled in size, about 1 hour 30 minutes.
4. Turn the dough out of the bowl, and knead briefly to release excess air. Re-form it into a ball, and return it to the bowl. Lightly butter a large piece of plastic wrap, and lay it on the dough. Cover the entire bowl tightly with the rest of the plastic, and refrigerate for 4 hours or overnight to let the dough proof.
5. For the filling: Preheat the oven to 350 degrees F. Spread the poppy seeds, sesame seeds, garlic flakes and onion flakes on a pie plate, and toast until lightly browned and fragrant, about 3 minutes. Let cool for 5 minutes. Mix the cream cheese, egg and flour in a small bowl until combined. Stir in the spring onions, lemon zest, 1 teaspoon salt and a few grinds of pepper.
6. To fill and form the buns: Butter a 9-by-13-inch baking pan with the 2 tablespoons softened butter. Turn the dough onto a floured work surface, and press it flat. Roll the dough into a 10-by-18-inch rectangle, with a long side facing you. Spread the filling evenly over the dough, leaving a 1-inch border on the side opposite you. Set aside 2 tablespoons of the toasted seed mix (for the topping), and scatter the rest over the filling. Starting from the side facing you, roll the dough up into a tight roll. Lightly brush the 1-inch border with water, and continue rolling to seal the roll. Fold the ends in just a bit, and if any spots look thicker than others, even out the roll with your hands.
7. Cut the roll every 1 1/2 inches with a serrated knife, to make 12 buns. Place the buns cut-side down (except for the end pieces) in the prepared pan, spacing them 1 inch apart. Cover the buns loosely with plastic wrap, and set in a warm place to rise until they double in size with no gaps in between, 1 hour 30 minutes to 2 hours.
8. For the topping: Stir the butter, reserved seed mix and spring onions in a small bowl; set aside.
9. Position an oven rack in the center of the oven, and preheat to 175°C. Bake the buns until they're golden brown and the tops spring back when pressed lightly, 25 to 30 minutes. Spread the topping evenly over the buns, sprinkle with 1 teaspoon salt and return to the oven until the butter is melted and the topping is fragrant, 5 minutes more. Let the buns cool in the pan for 10 minutes before serving.
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From Food Network Kitchen