Put the fruit juice in a saucepan and heat gently until quite warm but not yet boiling.
Sprinkle the agar over the surface of the liquid and continue to heat without stirring, while the agar dissolves.
When the liquid starts to just bubble, stir in the agar, then simmer, stirring occasionally, for a few minutes until all the agar flakes have thoroughly dissolved. (If you are using grenadine, add it now.)
Strain the hot jelly into four small serving glasses or dishes. Allow to cool slightly before transferring to the fridge until cold and set.
Before serving, sprinkle a tablespoon or so of pomegranate seeds onto the top of each jelly.