Stir-fry the onions with a tiny amount of the oil over medium-high heat for just a moment, to lightly char rather than cook the onion.
Add half the garlic and ginger, then the courgettes and peppers, and cook a few moments more or until courgettes are crisp yet tender.Remove from pan.
Add the remaining oil and stir-fry the mushrooms, adding the remaining garlic and ginger. Cook only for a few moments - they should remain crisp - then sprinkle in the stock cube and return the onions and courgettes to the pan.
Gradually stir in the water and the cornflour paste, cook for a few moments, and stir frying, until the liquid thickens, then season with soy sauce.
Serve immediately, with coriander scattered over on a bed of crispy noodles or steamed rice.