Rinse mushrooms and soak in water for about 30 minutes, or until soft enough that you can trim the stems. Rinse away any dirts or impurities. Change clean water and continue to soak until mushrooms completely softened. Drain out the mushrooms. Save the soaking water for later use. Use a handful of cornflour to mix with the mushrooms and rub with your hand. Set aside for 15 minutes, then rinse.
Use a deep pot, add the mushrooms, soaking water, rock sugar and a few drops of oil. Cook over medium-high heat. Bring it to a boil. Reduce heat to low and simmer until water dries up and thickens. Add more water if necessary along the way. Pour in the seasonings and stir to combine. Taste by yourself. Season with soy sauce or sugar if needed.
Meanwhile, blanch lettuce in boiling water with a bit of oil for 1 to 2 minutes. Don’t overcook, it should be a bit crunchy in texture. Drain well. Transfer on a serving plate. Arrange the mushrooms, and ladle sauce on top. Serve hot.