Blend the chopped strawberries and banana until completely smooth.
Pour the fruit purée into a saucepan and add the sugar, milk and cornstarch, whisking until smooth.
Cook over low heat, stirring constantly, until the mixture thickens and becomes glossy and smooth.
Remove from the heat, transfer to a bowl and cover the surface directly with plastic wrap, then cool completely.
In a separate bowl, whip the cold heavy cream to soft peaks.
Add the sweetened condensed milk to the whipped cream and gently mix until combined.
Fold the cooled fruit mixture into the cream mixture until smooth and evenly coloured.
Pour into a freezer‑safe container, cover and freeze for 4–6 hours, or until firm enough to scoop.
Let stand briefly at room temperature before scooping and serving.