Trim the berries and chop the rhubarb (it can be any ratio - I used just under a pound of berries and a couple small stalks of rhubarb) and put it into a medium saucepan with the sugar, lemon juice and salt. Bring to a simmer and cook, mashing the mixture occasionally with a potato masher or fork, until the fruit breaks down and it gets jammy. Scrape into a bowl and refrigerate until well chilled.
Make sure all your ingredients are cold, and stir together the fruit and creams in a bowl. Freeze in an ice cream machine until it turns into ice cream. Makes about 1 L.