If the sausage is in links, slit the casing and squeeze the meat out. Place the sausage meat in a small frying pan or wok. Cook over low heat until thoroughly cooked and the fat is rendered, about 15 minutes. Stir occasionally. Transfer to a sieve and press the meat with the back of a spoon to eliminate all fat. If the sausage meat is still in lumps, chop finely.
Soak the rice sticks in hot water for 20 minutes. Drain and cut into 2-inch lengths. Soak the mushrooms in hot water until soft, about 20 minutes. Discard the stems and shred the caps.
Combine the sausage, mushrooms, rice sticks, cabbage, bean sprouts, carrots, and green onions. Separately combine the sherry, oyster sauce, sesame oil, chili sauce, sugar, and garlic. Combine the cornstarch with an equal amount of cold water and set aside.
Place a 12-inch skillet or wok over high heat. When hot, add 2 tablespoons cooking oil; when the oil becomes hot, add the sausage mixture. Stir-fry until the vegetables are brightly colored, about 2 minutes, then add the sauce.
Stir-fry until the vegetables just begin to wilt, about 1 minute more. Add a little of the cornstarch mixture to thicken the sauce, then transfer to a bowl. Cool to room temperature, and then place in the freezer until thoroughly chilled, about 1 hour.
Separate the spring roll skins. Position each spring roll so one of the corners is pointing at you. Place about 1/2 cup of the filling in the bottom third of the skin and then form the filling into a cylinder, stretching between the side corners. Bring the corner nearest you over the center of the filling and then tuck the tip under the filling.
Roll the spring roll a turn. Brush the two side corners one-third over the top of the cylinder. Now finish rolling the spring roll into a cylinder.
Place on a small tray as you complete the rest of the spring rolls. Store, unstacked and uncovered, in the refrigerator.
Place 3 cups of cooking oil in a 12-inch skillet and heat over high heat. When the oil reaches 370 F (bubbles will escape from the end of a wooden spoon when placed into the oil for 10 seconds), fry the spring rolls in 3 batches, cooking them on both sides until light golden, about 3 minutes. Drain on a wire rack.
Heat the oil again, this time until it reaches 400 F. Fry the spring rolls a second time in 2 batches until they are dark golden, about 1 minute. Drain. The second frying makes the spring rolls crispier.
Serve spring rolls whole or cut in half, with one or more dipping sauces. Guest should wrap a spring roll in a lettuce leaf, dip it in sauce, and enjoy.