Mongo.find_one

You're seeing just the function find_one, go back to Mongo module for more information.
Link to this function

find_one(topology_pid, coll, filter, opts \\ [])

View Source

Specs

find_one(GenServer.server(), collection(), BSON.document(), Keyword.t()) ::
  BSON.document() | nil | {:error, any()}

Selects a single document in a collection and returns either a document or nil.

If multiple documents satisfy the query, this method returns the first document according to the natural order which reflects the order of documents on the disk.

For all options see Options

Use the underscore style, for example to set the option readConcern use read_concern. Another example:

 Mongo.find_one(top, "jobs", %{}, read_concern: %{level: "local"})