Register derived classes with manager class before instantiation

I have an abstract Spell class. Each spell the player can cast inherits from that Spell class, like Spell_Fireball : Spell.

I have a SpellManager class that should know about every spell. Since spells are just prefabs until they're cast, how can I make them register themselves with the SpellManager before instantiation?

My initial plan was to just have List<Spell> and then assign each spell prefab in the inspector, but I forgot Unity doesn't support serialising derived classes. Maybe I'm thinking about this the wrong way but I'm a bit stuck.