For the soup, melt the butter in a medium saucepan over a low heat.
Add the sliced onion, cover and gently cook until softened.
Increase the heat, add the wine and stock and simmer until reduced by a third.
To make the croutons, preheat the grill to the highest setting.
Drizzle the bread with the olive oil and season, to taste, with salt and freshly ground black pepper.
In a small bowl, mix together the grated cheddar, Dijon and season, to taste, with salt and freshly ground black pepper.
Heat a griddle pan until smoking. Add the bread and griddle for one minute on each side, or until lightly toasted.
Place the toast on a small baking tray, spread over the cheese mix and transfer to the grill to cook for two minutes, or until the cheese mix has melted.
Remove from the grill and cut the cheese on toast into chunky croutons.
To serve, pour the soup into a warm bowl and top with a few cheesy croutons.