prepare the dough
In a bowl, stir to combine lukewarm milk and active dry yeast. Set aside for 5 minutes. In a large bowl, stir to combine all-purpose flour and yeast mixture. In a separate bowl, whisk together egg, egg yolk, sugar, and salt using a hand whisk. Add the egg mixture to the flour mixture and knead it into a smooth dough. Add the melted (cooled) butter and knead it into the dough. Transfer the dough to a bowl, cover with kitchen cloth or clingfilm and set aside. Leave the dough to proof for 45 - 60 minutes.
make the filling
Add lard to a skillet and place over medium-high heat. When it melts, add the pork lard and fry for 4 - 5 minutes or until the pork lard is crispy and golden-brown. Pour the mixture over a fine-mesh sieve into a bowl. Reserve the cooking fat and place the crispy pork lard onto a separate plate. Clean the skillet, and add two tablespoons of the reserved cooking fat into the skillet along with the diced onion. Pan-fry for 10 minutes over low heat or until the onion is golden and soft. Finely chop the crispy pork lard and onion and add it to a bowl along with the sour cream and egg yolk. Stir and set aside.
roll out the dough and shape
Lightly dust the working surface with flour and roll the dough into a 50 cm x 35 cm or 20-inch x 14-inch rectangle, about 0.5 cm or 1/8-inch in thickness. Spread the filling over the dough, leaving a 1-inch (3 cm) margin at one long side of the dough. Start rolling the dough up, starting from the longer side (where the edge is), and place seam side down, making sure to seal the edges of the dough. Cut off about an inch or 2 cm off the ends of the dough, then cut the dough into about 1-inch or 3 cm sections. You should get 12 pieces.TipThe easiest way to cut the dough into sections is to use a kitchen string.
leave to proof
Line a 25 cm x 30 cm or 10-inch x 12-inch baking dish with parchment paper. Place the savory bread roll in the prepared baking dish. Cover with plastic wrap or kitchen towel and let rise at room temperature for 30 minutes or until doubled in size. Remove the plastic wrap and gently brush with lukewarm milk.
bake and serve
Place the savory bread rolls into the preheated oven on the middle rack. Bake for 30 - 40 minutes at 190 °C / 375 °F. When baked, remove from the oven and let cool to room temperature.