How could it be anything but circumstantial? It’s rare in any murder trial to have direct evidence, and poisoning is inherently a hidden and secretive crime. Unless she actually videoed herself picking poisonous mushrooms, putting them the meal, and saying she know’s they’re poisonous I can’t see any way you could have direct evidence.

Circumstantial evidence is not bad evidence. Once you have a reasonable amount of different bits of evidence all pointing in the same direction it becomes very solid evidence.