Separate the lettuce leaves & place them in a bowl of cold water so the leaves are submerged.
Heat the oil in a large frying pan over medium high heat. Cook the mushrooms in 2 to 3 batches until browned, adding a little more oil if necessary & removing cooked mushrooms from pan before adding more. When all the mushrooms are cooked, chop them finely. .
Add the garlic and ginger to the same pan with a little more oil if necessary. Cook, stirring for 1 to 2 minutes or until fragrant, still over a medium-high heat.
Add sherry to deglaze the pan. Cook, stirring until sherry is reduced by approximately half.
Add mushrooms, half the green onions, water chestnuts, hoi sin, soy & oyster sauce. Cook 1 to 2 minutes until warm.