Sweat the quartered mushrooms in a shallow saucepan. Stir little, otherwise the mushrooms will lose too much water. Pepper.
When the mushrooms are golden, add the onion cubes. If the onions are translucent, fill the ragout with the liquid cream and let the sauce reduce for 1/4 hour.
Season the mushrooms a la creme with balsamic vinegar, salt, pepper and sugar.