Bloom the yeast in the warm water for 10 minutes, then mix all the remaining dough incredient together in a bowl. Knead on a floured surface for 5 minutes, then cover and let rest 1 hour.
Make the filling. Heat the butter in a large pan and cook the cabbage, sauerkraut and onions over medium heat until soft, about 5 minutes. Add salt and black pepper to taste. Stir in the thyme, mustard, vinegar and beer and simmer until the liquid is all gone. Turn off the heat, and, when this is cool, mix well with the uncooked meat. Set in the fridge.
After the first hour has elapsed, punch down the dough, knead a few more times, then roll into a ball, cover and let sit another hour.
When the dough has about 30 minutes to go, take the filling out of the fridge. It is important that it goes into the bierock at room temperature. Preheat the oven to 350°F. Grease a baking sheet well.
Cut the dough into eight pieces. Set the pieces you are not working on under plastic wrap or a damp towel. Divide the filling into eight parts, too.
Roll a piece of dough into a ball, then flatten it to about 1/3 of an inch thick. Roll the outer inch of the dough thinner, to about ¼ inch thick or even thinner if you can – this makes what will become the bottom of the pastry match better with the top. Add some filling and bring the dough up all around it to seal. Set the dough seam side down on a floured surface and gently shape it into a flattened ball with your hands. Set on the greased baking sheet and repeat with the other pieces of dough.
When all your bierocks are made and on the sheet, paint them all with egg and sprinkle with the seeds, plus a little more salt.
Bake for 25 minutes, and cool a bit before serving. These are excellent at room temperature, too.