In a nonstick skillet over medium heat, brown the chicken. Add the mushrooms, garlic, and water chestnuts and saute for 3-4 minutes.
In a small bowl, stir together the scallions, soy sauce, oyster sauce, mirin, rice wine vinegar and chili garlic paste. Pour over the skillet mixture and stir until absorbed.
Heat a saucepan of vegetable oil to a depth of 1" to a temperature of 350F. Coarsely chop or break apart the rice noodles and working in batches, add to hot oil. After the rice noodles have puffed up and absorbed the oil, flip them over for 10-15 seconds. Remove to a paper-towel lined plate.
Spoon the chicken filling into a lettuce cup and top with rice noodles. Serve immediately.