1Sweeten raspberries with the sugar; stir in lemon juice.
2Beat with a mixer to break up the berries; keep icy cold.
3Melt butter in a saucepan, blend in flour and salt; add cream and milk all at once and immediately stir vigorously over moderate heat.
4Continue to cook until thickened, stirring constantly; cool, stirring occasionally.
5Chill; just before serving, add raspberries gradually to sauce, stirring vigorously.