Put the butter and olive oil into a large skillet and set over medium heat. Slice the onion into thin slivers and add them to the melted butter. Reduce the heat to low and cook the onions for 15 minutes until translucent. Add the mushrooms, rosemary, salt and pepper, and 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 3/4. Add the white wine and reduce again until the pan is almost dry. (The recipe can be made to this point the day before if you like. Reheat the mushroom mixture to continue with the recipe.) Add the crumbled gorgonzola and heavy cream. Bring to a simmer and reduce by half. Season, to taste, with salt and pepper, and spoon into the hollowed out boule. Garnish with a few rosemary sprigs and serve immediately with sliced baguettes for dipping.