In the current multiplayer examples, all newly created entities on the client & server have a GhostAuthoring component on the prefab.
How can the creation of a new entity (on the server) be synchronized without having a prefab with a ghost component?
Just send a custom RPC to the client with the required information.
That could work, but is there a way to sync RPC calls to a specific tick, so the action happens on the same simulation step on all clients?
You can write tick to RPC data and check it on the client-side.