To make the marinade, combine all ingredients together in a ceramic bowl. Add mushrooms, stir to coat. Cover and set aside for 1 hour.
Heat wok over high heat until very hot. Add 3 tsp oil and swirl to coat. Add mushrooms and stir-fry for 2 to 3 minutes or until glossy and almost tender. Transfer to a bowl.
Add remaining 1 tsp oil to wok. Add spring onions and capsicum. Stir-fry 1 minute. Add mushrooms and cashews and stir-fry 1-2 minutes or until heated through.