Automatically fetching and converting a recipe is "simple" as per your title. See here.
Run the command line on a scheduled script and you get the ebook saved to disk.
It becomes more complex when you want to add the new ebooks to Calibre to manage them with Calibre, or have Calibre manage them automatically as recipes by deleting older copies or autosend them to the ereader and delete them when sent, or just send them to the ereader automatically.
By the time your script does everything a sophisticated user wants it to do, it may be easier to just run Calibre and let it do its thing.