Discard the marinating liquid, lemon, and herbs and brown the hindquarters in a mixture of olive oil and butter over medium high heat in a pot suitable for braising.  After browning both sides of the hindquarters, remove them,  deglaze the pan with a little white wine and add the celery and onion.  After tossing the celery and onion, add back all the hindquarters to the pot, add enough chicken stock to just cover the rabbit, and braise over medium low heat.  Check the rabbit for tenderness at about an hour.  It will likely require more time to cook and become tender.  Check again every 20 minutes until the rabbit is tender.  (At this stage, when the rabbit has been braised to tender, you may serve it whole alongside polenta and vegetables, or continue with the sauce recipe below.  If you choose to serve the braised rabbit, remove from the pot and thicken the braising liquid over low heat to accompany the rabbit dish).

Remove the rabbit from the pan, reserving the braising liquid.  Pull the meat away from the bone and finely chop.  In a sauté pan, heat ¼ tbsp of unsalted sweet butter over medium low heat.  Once melted, add the chopped rabbit meat and toss with remaining finely diced sage and thyme.  Increase the heat to medium and begin adding back the braising liquid, along with the cooked onion and celery.  Add enough to create a sauce for 4 portions of pasta.  Salt and pepper to taste, and cook down to thicken until rich, adding flour if necessary (or pasta water if serving with fresh pasta, which is recommended). 
Serve the ragu with freshly made egg pasta, preferably a thick gauge spaghetti, tagliatelle, or fettuccine.  Use about a pound for 4 servings of pasta. 

The pasta pairs well with hearty whites, like the Vinchio Vaglio Serra Roero Arneis, "Il Griso", or the interesting reds of Piemonte such as Campo alla Sughera "Adeo" or Vinchio Vaglio Serra "I Tre Vescovi" Barbera d'Asti.  I served it last with a bottle of 2006 Orlando Abrigo Barbaresco, and it was fantastic.