1. Melt the butter and oil together in a large pan over a medium heat. Add the onions and salt and cook for 10 mins, stirring regularly until softened. Reduce the heat to low and cover the pan with a lid. Cook for 15 mins, stirring occasionally.

2. Add the garlic and cook, uncovered, for another 10-12 mins or until the onions are very soft and caramelised.

3. Sprinkle in the flour and stir to mix well. Increase the heat and pour in the wine, if using, allowing it to bubble away for a few mins. Make up the stock cubes with 1 ltr hot water and add to the pan along with the thyme sprigs. Bring to the boil, then leave to simmer for 30 mins.

4. Preheat the grill to high. Toast the baguette slices on each side. Ladle the soup into 4 heat-proof bowls or dishes and top each with 2 baguette slices. Scatter with the grated cheese then cook under the hot grill for 1-2 mins until the cheese is melted, bubbling and golden.

5. Garnish with freshly ground black pepper and a few fresh thyme leaves to serve.