Marinate the beef in sugar, soy sauce and cornflour slurry for 15 minutes.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry beef until 70% done, about 2-3 minutes. Set aside.
Heat the same frying pan with 1 tablespoon oil over high heat. Stir-fry the garlic until aromatic, about 2-3 minutes. Add the onion and salt; cook until soft, about 3-4 minutes.
Add the courgettes and 2 tablespoons of water and mix well. Cook until courgettes are tender, 2-3 minutes. Add the beef back into the pan with the oyster sauce. Stir and heat through for 1-2 minutes. Serve.