Melt butter in a large saucepan, add onion, sugar, season with salt and cook over low heat stirring occasionally for 25-30 minutes until very soft and browned
Stir mustard and flour into the onions, and cook for a further 3-4 minutes until flour is well browned, stir in wine and cook for a further minute before stock. Cover and bring to a gentle simmer for 25-30 minutes until thickened and dark, season to taste
In the meantime, spread bread with butter, top with sage leaves
Ladle soup into 4 ovenproof bowls or mugs, top with a slice of bread, sprinkle with parmesan and grill under a hot grill for 3-4 minutes until cheese is bubbling and bread is golden. Serve immediately
