Place the berries, sugar, sumac, and salt in a food processor. Pulse until it breaks down into a frozen purée, and stop just as soon as it’s pretty much smooth (don’t over-process). Remove the blade, and press the sorbet down into the bottom of the food processor with the back of a spoon.
It’s best served immediately, but you can store leftovers in a sealed container in the freezer (don’t let them thaw first, and be sure to freeze them right away). It sets up a little firmly in the freezer, but it softens to the perfect consistency after zapping it in the microwave for just a few seconds and then letting it sit for a minute (10 seconds in my microwave).
* Some bags of berries are 500g, in which case you’ll have some left for garnish. If not, no worries though! ** The sumac is here 100% for its tangy berry-heightening flavor—feel free to leave it out entirely if you want a plain mixed berry sorbet.