In a small bowl, mix together the oyster sauce, water, sesame oil, salt and sugar to make the sauce. Set aside.
Blanch bean sprouts and green beans in boiling water for 1-2 minutes. Pour into a colander set in the sink and cool with a cold running tap. Drain and set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the garlic until fragrant, 2-3 minutes. Add the onion with 1 teaspoon salt, stir-fry until soft, 3-4 minutes. Stir in the ham and mix well. Add the sauce and heat through, 1-2 minutes. Serve.