Bring the chicken stock to a boil and add the pasta. Stir now and then as the pasta cooks. In the meantime, heat a large frying pan and add the chicken fat and the onions. Sauté for a few minutes and add the mushrooms. Sauté for a few additional minutes, until the mushrooms are tender. Once the pasta is just tender, drain and add it to the frying pan. Add salt and pepper and serve hot. Serves 6 - 8 as a side dish