Preheat the grill to to its highest setting.
Heat the oil in a saucepan, add the onion and garlic and cook for five minutes, on a low heat, then turn up the heat for 2-3 minutes, to brown the edges of the onion.
Add the stock, soy sauce, Worcestershire sauce and caster sugar, allow to simmer, until thickened.
Meanwhile, top the ciabatta slice with the cheese and place under the grill to cook for 1-2 minutes, until bubbling and golden.
To serve, spoon the soup into a bowl and top with the crouton.