In a medium skillet, heat sesame and salad oil. Sauté mushrooms for 3 minutes. Add ginger, choi sum, water, and soy sauce; stir-fry for 1 minute. Remove from heat; add bean sprouts and green onion. Set aside to cool. Beat eggs and milk. Melt 1 tablespoon of the butter in an 8-inch non-stick pan. Slowly pour in 1/4 of the egg mixture. Tilt the pan to spread the eggs evenly; cook until the mixture holds together. Add 1/4 of the choi sum mixture to the center. Tilt the pan to one side, and with a spatula, fold about 1/3 of the omelet over the middle. Shake the pan gently to slide the omelet to the edge of the pan. Turn the omelet directly onto the plate by inverting the pan over the plate. Brush top with oyster sauce and garnish with sesame seeds. Repeat until all of the egg and the choi sum mixture is used. Makes 4 servings.