What is the best way to thicken a light fruit syrup from preserved berries to make it stick, with the berries, as a topping on a cheesecake.

Thanks, everyone. I ended up draining the berries and simmering the syrup to reduce to about 1/4 of its original volume. It was a bit anemic, even when reduced, so I added a scant tablespoon of sugar and simmered until sort of syrupy. The berries were tart, small ones, so the syrup could tolerate just a bit more sugar. Then I added the syrup back to the berries, let it sit for an hour or so, then gave it a good stir and put it all on top of my "Corsican Ricotta Cheesecake", adapted from a Patricia Wells recipe. It worked perfectly! ;o)

I'd suggest setting it with gelatin, as for a miroir. That way, you don't have to overcook the fruit or risk scorching anything. And the finish will have a lovely shine.

I'd reduce it down in the microwave to minimize risk of burning as per Rose Levy Beranbaum's advice.

Sorry! Meant to say, if you use cornstarch, take a bit of the juice out, let it cool a bit and slake about a teaspoon in, then add that to the juice. You'll need to bring it to the boil to thicken it, you can always add more, it depends on how thick it is to start with.

Cornstarch will give you a cloudy finish, which isn't really a problem, but the usual thing to use is arrowroot powder, as it keeps the juice clear.

The syrup would be thin owing the higher percentage of water, so you can just reduce it by simmering the liquid - no cornstarch needed.

Getting the right consistency takes some trial & error, because the sauce thickens as it cools. Boil it for a bit to reduce, allow it to cool, test it and boil again if too runny. If too thick, mix in a bit of water.

You can use cornstarch (not very much! and make sure to boil it if you do to cook out the starchy taste) but this method works just as well and may give a cleaner taste on the palate.