Melt butter in a skillet over medium-high heat. Sauté onion in butter until tender. Remove from heat, and set aside to cool. When the onion is cool enough to handle, mix together with ground pork, bread crumbs, egg, salt, and pepper until well blended.

Place head of cabbage in a large pot, and add enough water to cover cabbage halfway. Bring to a boil, and cook for about 2 minutes. As the outer leaves become translucent and soft, remove them one at a time by peeling them off with a wooden spoon. Trim off the thick ribs at the leaf stem.

Preheat the oven to 325 degrees F (165 degrees C). Grease the bottom of a 9x13-inch baking dish, and line the bottom with outer cabbage leaves.

Lay a few more leaves on a clean work surface. Place a large spoonful of the pork mixture at the base of each leaf. Roll up towards the top. Repeat until filling is used up. Arrange the finished rolls over the leaves in the dish, and pour in just enough chicken stock to cover the rolls. Place extra cabbage leaves over the top.

Cover, and bake for 1 hour in the preheated oven.

Drain off cooking liquid from the dish into a small saucepan. Whisk together sour cream and flour. Gradually stir the sour cream into the broth. Cook over medium heat, stirring frequently until heated, about 5 minutes.