Heat up the olive oil in a large skillet and caramelize the onions using this Caramelized Onions Recipe. Once the onions are caramelized add the sliced mushroom and sauté them together for about 5 minutes until softened. Stir in the parsley.
Meanwhile start the polenta: bring the water or milk to a boil then pour in the corn meal in a slow stream while whisking constantly. Add the bay leaf and a pinch of sea salt and allow the polenta to gently simmer away on low flame for 45 minutes, until thick and creamy. Stir often with a wooden spoon making sure it doesn’t stick. Remove from flame, discard the bay leaf and stir in the cream. Adjust seasoning to your taste and whisk in the truffle butter. Allow it to set while you preheat the broiler.
Oil the bottom of a medium size, broiler proof gratin dish. Pour half of the mushroom and caramelized onion mixture on the bottom of the pan and top with the creamy polenta. Add the remaining of the mushroom and onions on the top and sprinkle with the cheese. Finish it up under the broiler flame until the cheese has melted and the mushrooms start to brown, 2-3 minutes. Serve hot as a side dish or main course.