Rinse the mushrooms well, cut off and discard the ends of the stems, and slice them about ¼ inch thick. Set aside.

Put the butter and olive oil into a large skillet and set over medium heat.

Add the onion to the pan. Reduce the heat to low and cook the onions for 15 minutes until translucent. Add the mushrooms and rosemary, raise the heat to medium, and cook, stirring occasionally, until the mushrooms give up their liquid. Let the mixture simmer until the mushroom liquid has reduced by three quarters. Add the white wine and reduce again until the pan is almost dry.

Add the Gorgonzola and cream. Bring to a simmer and reduce by half, stirring occasionally. Stir in the salt and pepper and spoon into the hollowed-out boule. Mound sliced baguettes around the boule and serve immediately.