Let the berries thaw, put them in a saucepan with the preserving sugar, a little lemon juice and the ground vanilla, stir well and let it steep for a moment.
Bring the mixture to the boil, make a jelly test and add the chia seeds. Stir in and let simmer for about a minute.
Pour into a hot rinsed glass, close the glass and let it stand upside down for a moment.