Preheat oven to 200°c. Line a baking tray with baking paper.
Melt butter in a large saucepan over medium heat. Add onion and thyme and cook for 10 minutes, stirring occasionally. Add sherry and stock and bring to the boil. Season with salt and pepper to taste.
Meanwhile, place the bread on prepared tray and sprinkle with cheese and extra thyme leaves. Bake for 5 minutes or until cheese is golden. Pour soup into bowls and serve immediately with hot cheese toasts.