WikiPedia says that relish is “a cooked, pickled, or chopped vegetable or fruit food item which is typically used as a condiment.”
So, technically, that jar of chopped pickled cucumbers you smacked Alex alongside the head with is not a relish, as it was used as a weapon, not a condiment.
However, if you decide to decapitate Alex, cook his head, and serve it with the chopped pickled cucumbers on the side, then it’s still not relish.
Chopped pickled cucumbers do not go with cooked severed heads at all. However, based on its presentation, you could call it a garnish.