For the salad onions, place the salad onions and olive oil into a bowl. Season, to taste, with salt and freshly ground black pepper and toss together.
Heat a griddle pan until smoking. Add the salad onions and char-grill for two minutes on each side, so that brown griddle marks are formed. Remove from the heat and set aside.
For the noodles, heat the sesame oil in a large wok over a high heat. Add the garlic, chilli, salad onions and aubergine and stir fry for one minute.
Add the spinach and prawns and cook for a further two minutes, turning the prawns halfway through cooking.
Add the noodles, drizzle over the soy sauce and toss together.
To serve, place the noodles on a large warm plate and sprinkle with sesame seeds. Place the char-grilled salad onions criss-crossed on top.