Wash and hull the strawberries leaving them whole. Place in a serving dish and chill while preparing the sauce.
Place the raspberries in a saucepan. Add half the sugar and heat very gently for about 5 minutes, crushing the fruit a little to make the juice run. When the fruit is soft, rub through a sieve to make a purée and discard the pips. Add the remaining sugar and the strained lemon juice. Pour over the strawberries and toss to coat the fruit well. Serve chilled.