Preheat the oven to 375°F. Warm the milk. Add the yeast and allow the yeast to bloom for 10 minutes. Meanwhile, put the butter in a sauté pan over a medium heat and sauté the onion. Remove the sautéed onion from the heat and allow it to cool to room temperature. Put the bloomed yeast in the bowl of standing mixer. Add the rye flour, bread flour, and salt and start to bring the dough together with a dough hook. Add the pig’s blood, sugar, molasses, and onion. Mix the dough until it becomes smooth. Allow the dough to double for 20 minutes. Divide the dough into 8 loaves. Poke the loaves with a fork all over. Proof for 5 more minutes. Egg wash and bake for 20 minutes.