Slice the onions, then add the butter and oil in a deep-bottomed pan
Add the onions, season with salt, mix well and sauté on a medium heat until softened (15-20 minutes - stirring regularly to avoid burning)
Add the sugar, then cook for a further 10-20 minutes until golden and soft
Mince the garlic and sauté until fragrant
Add the plain flour, stir well until incorporated
Increase the heat, then add the wine gradually to deglaze the pan, stirring well as you go
Add the hot vegetable stock, rosemary, thyme and bay, then simmer for 30 minutes
Slice the baguette and toast under the grill with a splash of oil or in toaster
Ladle the soup into heatproof bowls, put the bread on top, then cover with grated cheese
Place under the grill until melted, then top with finely chopped chives and enjoy
