Melt the butter in a medium saucepan over a low heat.
Add the sliced onion, cover, and cook until softened.
Increase the heat, add the wine and stock, and simmer until reduced by a third.
Preheat the grill to the highest setting.
Drizzle the bread with olive oil and season with salt and pepper.
Mix the grated cheddar and Dijon mustard in a small bowl and season.
Griddle the bread for one minute on each side in a smoking hot pan.
Spread the cheese mixture on the toast and grill for two minutes until melted.
Cut the cheese on toast into chunky croutons.
Pour the soup into a warm bowl and top with the croutons.