Mushrooms, beef, long cooking and bitterness I tried two long-cooking beef dishes starring mushrooms, an ossobuco of veal and beef breast.
The directions went by basically cooking the ossobuco (after cutting the junctions) or the beef breast cubes in vegetable stock, not submerged, for some hours until tender.
Then I was supposed to add mushrooms and cook for another while. Champignon in the case of ossobuco and girolle in the case of the beef breast stew.
They did hold their shape after at least an hour more, but the resulting dish was in both cases bitter. How can this be related to the mushrooms?
I can't identify other possible causes for this. Both dishes included also a splash of wine, but in both cases, it had not turned into vinegar. The stew did contain a bit of rosemary but dry (and the ossobuco was way bitter anyway).
The cooking vessel was a steel pan for the ossobuco and a cast iron pot for the stew.
Ooooh!
I would not recommend using cast iron for slow cooking or long cooking of any kind. Especially if any sort of acidity is introduced. That bitter taste? That straight up might be the taste of the pan corroding. Cast iron is great for fast frying meats, sauteing vegetables, but it is not recommended that you make anything that has much liquid, and definitely avoid acidic foods. This is why it is always important to wash and dry your cast iron as soon as you are finished cooking in it - to avoid rusting and peeling.
Additionally.... I would try sauteeing your fresh mushrooms first in a bit of butter, over high heat. you want to sear them. Then add them to your soup :) the full umami (salty) flavor should come out nicely!