Python Open Closed Principle when your objects could be a list of instances?

I have a number of different objects that I can 'jsonify', these are custom object types:

class Jsonable(ABC):
    @abstractmethod
    def extract_json():
        pass # return json of self

class Organization(Jsonable):
    # implements the abstract method, all good here
    def extract_json():
        return # some json of self, great!

class Feature(Jsonable):
    # implements the abstract method, all good here
    def extract_json():
        return # some json of self, great!

I have a function, where I want to pass in many different types of 'Jsonable' and get the json for them, but theres a catch, the 'str' class is a valid type to this function, and also a List[Jsonable] is also valid, how do I have a clean function for returning the data?

def extract(data: Union[Jsonable, List[Jsonable], str):
    if isinstance(data, str): 
        # do something about string
        # not great but I can live with this, it will never change
    return data.extract_json() # ok for the standard types (Org above)
    # what about List[Jsonable]?
    # I have many types, Organization above is one example

How do I make this extract function not violate the OCP and get a nice clean way to abstract the data from these types? I should be able to get the json from the listed types aswell, cleanly?

The List can't really extend Jsonable, so how do I handle this cleanly?