This recipe takes about 2 minutes:
Take a frozen punnet of berries (strawberries, blueberries, raspberries, cherries, black currants, etc.)
Place them in a good quality blender (it is going to have to work hard, and you don't want to burn out the motor)
2 parts frozen berries, to
1 part double cream (aka Heavy Cream in America)
and some sweetener (judging the sweetener will take a bit of practice, and will depend on the brand)
Blitz.
Serve immediately (it will set rock solid if you try and freeze it for later)
A little practice and you can adjust the sweetness and the softness/hardness by tweaking the proportions.
It can also be done with frozen milk, chocolate pieces, and all sorts of other low carb ingredients.