Preheat an oven to 375 degrees F.
In a medium mixing bowl, combine the hard boiled eggs, parsley, onions, bread crumbs, melted butter, and enough broth, as needed, to make a wet stuffing. Pat the sweetbreads dry with a paper towel and make a pocket in the middle of the sweetbreads big enough to fill. Season the sweetbreads with salt and pepper and fill cavity with stuffing mixture. Heat the grapeseed oil in a medium skillet with an oven safe handle. In a shallow bowl, season flour with salt and pepper and dredge sweetbreads in flour, shaking off excess. Gently transfer sweetbreads to hot skillet and sear for 3 to 4 minutes per side. Place the whole pan in the preheated oven to finish. Test for doneness after about 5 more minutes by touching the sweetbreads to see if the interior has "tightened up" enough so it will remain intact when it is sliced. When this point has been reached remove from oven and let rest for about 20 minutes to allow all the flavors to flow back into the sweetbreads and allow it to set up before slicing.