How are you running the command? Since prodigy.serve is already executed from Python, it shouldn't need to point to another file containing the recipe script. That's only needed on the command line using the -F flag.
If you're already within a Python script, you should be able to just include the recipe script before the call to prodigy.serve (either in the same file, or imported from a module – however you like to structure your code). The @recipe decorator registers the recipe with Prodigy, and when you call prodigy.serve with the name, Prodigy should be able to find it.