Make basic white sauce with the butter, flour and milk: Melt butter
over medium heat; when bubbling, remove from heat, add flour and work
it into a loose paste. Add cold milk gradually and whisk until
smooth. Return to heat, cook over low heat, stirring almost
constantly, until thickened and starch taste has disappeared.

Alternatively, you can do this in the microwave, which eliminates all
that stirring. Melt butter on high, about 90 seconds. Add flour,
then milk, as above. Cook on 70% power, until desired consistency,
whisking every 3 minutes. Should take about 10-12 minutes.

While the sauce is doing, break the tomatoes up into bite-size pieces,
then warm them gently in a saucepan (at least 2.5 litres), with
generous sprinkles of sugar, freshly ground pepper, your choice of
herbs (basil, thyme, tarragon, dill are all good) and up to 1 tsp dry
mustard. When the tomatoes are warm and the sauce completed, add the
sauce to the tomatoes, stir well, and continue warming until piping
hot.