Remove the stalks from 300g fresh strawberries and roughly chop them.
In a small saucepan, combine the strawberries with 25g caster sugar and 15ml water. Cook over medium heat, stirring occasionally, for 5-7 minutes until thickened. Mash lightly to make a compote, then leave to cool completely.
Put 100g buttery shortbread biscuits into a plastic bag and crush into coarse crumbs. Melt 25g unsalted butter, pour over the crumbs and toss to combine. Set aside.
Pour 480ml cold double cream into a large, chilled bowl. Using an electric hand mixer, whisk on medium-high speed until stiff peaks form, about 3-5 minutes.
Gently fold in a 397g tin of sweetened condensed milk until just combined.
Spoon half the mixture into a 23 x 13cm loaf tin and spread evenly.
Dollop half of the cooled strawberry compote over the top, then sprinkle with half of the shortbread crumbs.
Repeat with the remaining mixture, strawberry compote and crumbs. Swirl lightly with a knife or cocktail stick to create distinct ribbons.
Cover the tin tightly with cling film and freeze for at least 6 hours, preferably overnight, until firm.
Before serving, leave the ice cream at room temperature for a few minutes to soften slightly, then scoop into bowls or cones.