Heat butter and oil in a large pan and fry onion over a low heat for 30 minutes, stirring occasionally until caramelised. Add flour and stir for 1 minute. Add stock and stir until smooth. Simmer for 30 minutes.
Cut bread into triangles and toast one side. Sprinkle with cheese and grill to melt. Ladle soup into bowls and serve topped with two toasts.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.
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