Chop the onions and garlic finely. Cook them for 40 minutes over low heat in a heavy pan in butter and oil to which caraway and pimenton have been added. (After 20 minutes put in the chopped bone marrow.)
Meanwhile poach the chicken breasts in the stock for 15 minutes. Chop them.
When the onions and garlic (and bone marrow) have melted, add the mushrooms and, as they are beginni