Using Radiant Destruction for potion-making was a novel idea for Lensa.
She was amazed by how an Ability can be put to use in various fields instead of just being used on one's opponents for their destructive properties. She started looking at her Ability from a fresh point of view.
The potion she was creating was supposed to have healing properties. However, the moonlight refined by her ice-element lenses gave it a special property. An ordinary raw ingredient was transformed into a special one.
Eren and Lensa were uncertain what kind of results it would produce. But they were hoping that the final product would be better than the estimated result usually derived from the same recipe.
The elf prodigy controlled the ice-element lenses through her Ability and did what Eren asked him to do. The latter paid close attention to her timing as well as the other aspects of the recipe to make the concoction a success on the first try.