Blanch the bean sprouts for 30 seconds in boiling water. Drain and set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Saute the garlic until aromatic, 2-3 minutes. Add the spring onions, onion and salt; cook until soft, 3-4 minutes.
Stir in the capsicums and mix well. Add the duck, beansprouts, hoisin sauce, oyster sauce and water. Stir and heat through, 2-3 minutes. Serve.