Line a 1-litre loaf pan with 2 sheets of plastic wrap, leaving an overhang on all sides to make unmoulding easy.
Process the strawberries in a food processor till well puréed. Pour the berries through a coarse strainer and extract as much liquid as possible.
Bring sugar, water and salt to a boil in a small saucepan over high heat, stirring until sugar has dissolved. Boil for 2 minutes and remove from heat. (Just a rough gauge will do, it will be lightly golden in colour.)
Beat egg whites on medium speed till foamy. Increase the speed to medium-high and pour in the hot sugar syrup into the egg whites with the mixer running. Be careful to avoid the beaters or else the syrup will splatter out. Continue beating for 10min till the egg whites are cooled to room temperature.
Beat the cream till stiff peaks form. With a whisk or a rubber spatula, fold berry purée into the egg white mixture, then fold in the whipped cream.
Spoon into ice cream container and spread into the corners with a rubber spatula. Cover with the overhanging plastic wrap or another piece of plastic wrap directly on top of the berry mixture. Freeze for at least 6 hours or overnight.
Turn out the semifreddo onto and serving plate. Let it soften for 10min before cutting into slices and serve with additional berries