- Peel and chop onion finely.
- Melt butter in a saucepan and gently fry onion for 2 minutes.
- Add tomatoes and stir until heated through.
- Add tomato paste, mustard, Worcestershire sauce and water.
- Bring the mixture to the boil and simmer for 30 minutes, stirring occasionally.
- Blend the cornflour with a little water and stir into sauce.
- Continue stirring until the sauce boils and thickens.
- Season with salt and pepper.