Mushroom Ragu over Cheesy Polenta

Ingredients:

1/2 oz. dried porcini mushrooms

3 tablespoons butter
2 tablespoons olive oil
3 medium red onions, sliced  

salt
2 lb. fresh mushrooms trimmed and chopped
1/2 cup dry vermouth
2 tablespoons heavy cream
1/4 cup mixed fresh parsley, thyme and sage
black pepper
1 cup polenta
1/2 oz. grated parmesan