Combine the mustard, flour and sugar with about a tsp of salt and a good grinding of pepper in a heavy based saucepan.
Add a little of the milk and stir to mix to a smooth paste, then put on a gentle heat and keep adding the milk, stirring as you go.
When all the milk has been incorporated, add the beaten egg and vinegar and keep on whisking until its beginning to thicken. When it has the texture of single cram whisk in the oil then plunge the saucepan into the sink of cold water and keep whisking for a while.
When it is cool cut the tomatoes in half and arrange on a large plate. Drizzle over a few spoonfuls of the salad cream then sprinkle over the chopped spring onion or chives.