1. Pour about 1 inch oil into a 14-inch wok or a 5- to 6-quart pan over high heat. When oil reaches 375°, add noodles in 2 equal batches, turning over once until they puff, almost immediately. Lift from pan with a slotted spoon and drain on paper towels.
2. In a 10- to 12-inch frying pan over high heat, stir and crumble beef until small pieces. Add mushrooms, water chestnuts, green onions, 2 tablespoons soy sauce, and chili flakes. Stir often until juices evaporate and mixture is lightly browned, 5 to 7 minutes.
3. Meanwhile, in a small bowl, blend cornstarch and sherry until smooth. Add to pan and stir until mixture boils. Remove from heat and add more soy sauce to taste. Pour into a bowl. Put noodles and lettuce in separate bowls.
4. For each portion, spoon noodles and meat mixture onto a lettuce leaf and roll up to eat.