Slice chicken thinly across the grain. Combine the lemongrass and 2 teaspoons oil in a bowl. Add the chicken and stir to coat. Cover and refrigerate for 10 minutes.
In a bowl combine soy and sherry, add mushrooms and stir to coat. Cover and set aside 10 minutes.
Heat a wok over high heat until hot. Add 1 teaspoon oil and half the chicken, stir-fry for 2 minutes or until golden. Remove to a plate and repeat with 1 teaspoon oil and remaining chicken. Remove and set aside.
Reheat wok until hot, add remaining oil. Drain mushrooms and add to wok, stir-fry 3 minutes until golden. Add asparagus and snow peas, stir-fry 1 minute. Add water chestnuts and chicken. Cook for 1 minute until warmed through. Remove from the heat, add the spinach, toss to combine. Serve with rice.